Evidence from computers – the unreliable legal presumption that, without more, it can be relied upon

By Stephen Mason and Daniel Seng, editors, Electronic Evidence and Electronic Signatures (5th edition, Institute of Advanced Legal Studies for the SAS Humanities Digital Library, School of Advanced Study, University of London, 2021)

 On 16th July I got a fish supper from the Harbour Café, in Girvan, Ayrshire. I paid by @BankofScotland debit card. My statement says I was at the Harbourside Café in Lynmouth, Devon. 450 miles away. It could have been an interesting alibi. “I was in Devon. The bank confirms it.” @james_christie, Twitter, 3:21 PM · Aug 22, 2022.

The editors of The Barrister kindly published an article of mine in 2008.[1] I attempted to introduce the importance of electronic evidence to lawyers by referring to the first edition of Electronic Evidence: Disclosure, Discovery & Admissibility (2007). I included a brief discussion on presumptions at 8.53 to 8.56 in the book. Had I been aware of the prosecutions by the Post Office over the Horizon computer system when researching this practitioner text, I would certainly have spent more time understanding the topic in greater detail.

My epiphany occurred when the Bar Pro Bono Unit instructed me to act for Mr Job in his action for the claimed loss of £2,100, withdrawn via ATMs. The trial took place before His Honour Judge Inglis in the Nottingham County Court on 6 May 2009.[2] I knew that Counsel for the bank would argue that the banking systems were presumed to be reliable. All I had at my disposal was the page and a half of my research. I took photocopies of the text, knowing that if I merely cited the book, neither the judge nor opposing Counsel will be aware of the relevant discussion, possibly will not have read the book – and not even be aware that the book existed (which indeed was the case).

Although the judge accepted my submission that one should be wary of taking the presumption at face value, I left this trial with a mission: to thoroughly research the law and technical side of computers to enlighten myself and the legal profession. Hence the second edition (2010) included a separate chapter on the presumption. This chapter has been improved since, and is the only detailed discussion of the topic, both legally and technically, available to lawyers and judges.[3] As a result of the concern relating to this presumption, a number of those reviewing this chapter agreed to set up an informal ‘Evidence Critical Systems Working Group’ (comprising IT specialists and lawyers) that remains active.[4]

The Law Commission proposal [Heading]

In 1997, the Law Commission published a paper Evidence in Criminal Proceedings: Hearsay and Related Topics.[5] Computer evidence was considered in Part XIII. Reviewing the problems faced by prosecutors, the Law Commission considered the law to be unsatisfactory and expressed its view that PACE 1984 s69 served ‘no useful purpose’.[6] It proposed that s69 should be repealed (and not replaced) with the effect that:[7]

‘In the absence of evidence to the contrary, the courts will presume that mechanical instruments were in order at the material time.’

The Law Commission considered that the words ‘mechanical instruments’ would extend (by default) to include computers,[8] and recommended that the presumption be reinstated for reasons of practicality whilst believing (wrongly) that justice would not be affected. The law was subsequently changed.[9]

The mischief [Heading]

The consequence of its repeal, in practice, has been to reverse the burden of proof placing it upon a defendant to demonstrate why the output from a computer should not be relied upon as a true statement (of fact). The presumption effectively ‘magics’ computer output into truth – a privilege rarely accorded other species of hearsay evidence. This need not have been the result. The practical issues that arise with evidence in digital form will always be with us. A recommendation to resolve this issue has been made to the Ministry of Justice at their request.[10]

The Law Commission supposed that repeal would not impose too onerous a burden on a defendant.[11] In practice, the supposition (that was little other than an assumption) is wrong and it is too onerous (and often insuperable) a burden. By implication, the Court of Appeal recognised this in Hamilton, although they recognised the reversal of burden of proof problem while not drawing any conclusions.[12]

The finder of fact acts as a moral agent [Heading]

The proponents of the presumption that computers are reliable failed to provide evidence that the presumption has any basis in fact.[13] The lack of any evidence to support the proposition is especially relevant in the light of the underlying rationale of evidence. Professor Hock Lai Ho demonstrates that the finder of fact is a moral agent,[14] and central to this is that the findings by a court must be justifiable, and meet the demands of rationality and ethics:

‘The court must not only find truth in order to reach a just outcome, it must also do justice in the course of ascertaining the truth’ (p 49).

‘The trial – or more specifically, trial deliberation – seeks the truth. It seeks the truth via justified belief in the facts of the case.’ (p 171).

In discussing hearsay: ‘The trial may be seen as a process of acquiring the moral authority to insist that the person whose case is before the court accepts its decision.’ (p 283).

The way the presumption has been dealt with in England & Wales (as exemplified by the Post Office Horizon litigation[15]), illustrates the point made at 5.249 of the current edition of Electronic Evidence and Electronic Signatures:

‘When read in the light of the unique characteristics of evidence in digital form, the rationale of the evidential process takes on an even more relevant role. This is because the factors and subsequent analysis have an added poignancy when taking into account the complexity of electronic evidence: the potential volumes of evidence, the difficulty of finding evidence, persuading the judge to order additional searches or to order the disclosure of relevant digital data, the ease with which electronic evidence can be destroyed, the costs of such exercises, the lawyer’s lack of knowledge when dealing with this form of evidence and the presumption that computers are ‘reliable’ or ‘working properly’. In this respect, the inadequacy of the procedure leading to trial brought about by an incomplete understanding and application of the presumption may cause unfairness.’

Will the legal profession enter the twenty-first century? [Heading]

Unless this presumption is reconsidered as a matter of urgency, we will witness further miscarriages of justice.[16] It is arguable that the Post Office scandal suggests more widespread injustice. It only emerged because of massive financial resources allocated to a class action brought by 550 claimants. The ‘reliability’ of computers has been more than adequately discussed,[17] and recommendations have been made to the Ministry of Justice (noted above), and suggestions have been offered about how the legal system can approach evidence in electronic form.[18] Other initiatives are in train,[19] including a review by the Law Commission of the law governing appeals in criminal cases, including appeals against conviction and sentence, with a view to ensuring that courts have powers that enable the effective, efficient and appropriate resolution of appeals.[20]

But without education (in addition it is necessary to revise the presumption), I fear lawyers will continue to be considered fit to advise the public without adequate knowledge of this important facet of legal practice.[21] Lawyers should be equipped with sufficient technical proficiency to deal with routine cases involving computer evidence, but also the knowledge needed to recognise when a case is not routine and that an expert needs to be appointed. For both scenarios lawyers would need to be able to ensure sufficient disclosure. Evidence in electronic form now comprises every item of litigation. Why is it right to certify that a person is qualified to advise and represent members of the public without even a basic understanding of this topic in the twenty-first century?

The opening quote shows how an alibi supported by apparently reliable electronic evidence could be deceptive. Expert evidence on the quality or otherwise of the electronic evidence would be fraught with difficulties, since virtually the entire electronic system may in fact be working reliably, and an expert could provide evidence that “everything” works. In reality, the error might or might not be a problem in the coding interface; or a problem in the café; or the EPOS system; or the bank; or Visa; or Google; or Google’s databases; or the cafés’ naming or registration with Google; or any other service the bank uses; or any intermediaries, or any combinations thereof. It might be field truncation. It might be errors in cryptographic interfaces. It might be due to an incompatibility with two or more connected systems, neither of which separately has problems. It might even be malicious hacking and nothing to do with the systems as such. It might be human operator error. The point is, everything may appear to be working perfectly, yet the electronic evidence actually has no probative value.

The Ayrshire company has no disclosed links with Lynmouth – the proprietors live in Ayrshire.[22]

© Stephen Mason, 2023

[1] Stephen Mason, ‘Digital evidence: the new challenge’, The Barrister, Number 36, 1 April – 23 May 2008, 18.

[2] Job v Halifax PLC (not reported) Case number 7BQ00307, 6 Digital Evidence and Electronic Signature Law Review (2009) 235,

[3] Stephen Mason and Daniel Seng, editors, Electronic Evidence and Electronic Signatures (5th edition, Institute of Advanced Legal Studies for the SAS Humanities Digital Library, School of Advanced Study, University of London, 2021

[4] I thank the members of the group for commenting on various iterations of this article before submission to the editors of The Barrister.


[6] Paragraphs 13.7-13.12.

[7] Paragraph 13.13.

[8] Paragraph 13.14.

[9] Section 69 of PACE 1984 was repealed by the Youth Justice and Criminal Evidence Act 1999. The result was that the law makes the presumption that the Law Commission identified and recommended.

[10] This article sets out the recommendations made to the Ministry of Justice at the request of Mr Alex Chalk MP, then Parliamentary Under-Secretary of State: Paul Marshall, James Christie, Peter Bernard Ladkin, Bev Littlewood, Stephen Mason, Martin Newby, Jonathan Rogers, Harold Thimbleby, Martyn Thomas CBE, ‘Recommendations for the probity of computer evidence’, 18 Digital Evidence and Electronic Signature Law Review (2021) 18,

[11] ‘We believe, as did the vast majority of our respondents, that such a regime would work fairly’, paragraph 13.23.

[12] Hamilton v Post Office Ltd [2021] EWCA Crim 577,

[13] Rather than citing technical experts in support of repeal, the Law Commission repeatedly quoted vague, arm-waving, un-evidenced comments by judges who offered no insight into anything beyond their own technical ignorance. Where the Law Commission does cite experts, the quotes did not offer any support: quite the reverse – they undermined the case for repeal.

[14] A Philosophy of Evidence Law: Justice in the Search for Truth (Oxford University Press, 2008).

[15] For an introduction, see ‘The Post Office Horizon Scandal a brief chronology’,; the transcript of the second trial is published in full in the 2021 edition of the Digital Evidence and Electronic Signature Law Review, and includes the two judgments by Coulson LJ regarding Bates v Post Office Ltd (No 4: Recusal Application) and Bates v Post Office Ltd (No 3: Common Issues), neither of which are published anywhere else.

[16] For articles dealing with the consequences when relying on the ‘reliability’ of computers, see James Christie, ‘The Post Office IT scandal – why IT audit is essential for effective corporate governance’, 19 Digital Evidence and Electronic Signature Law Review (2022), 42,; Paul Marshall, ‘Scandal at the Post Office: The Intersection of law, ethics and politics’, 19 Digital Evidence and Electronic Signature Law Review (2022), 12; Paul Marshall, ‘The harm that judges do – misunderstanding computer evidence: Mr Castleton’s story, 17 Digital Evidence and Electronic Signature Law Review (2020) 25,; Tim McCormack, ‘The Post Office Horizon system and Seema Misra’, 13 Digital Evidence and Electronic Signature Law Review (2016) 133,

[17] James Christie, ‘The Post Office Horizon IT scandal and the presumption of the dependability of computer evidence’, Digital Evidence and Electronic Signature Law Review 17 (2020) 49,; Peter Bernard Ladkin, ‘Robustness of software’ Digital Evidence and Electronic Signature Law Review 17 (2020) 15,; Peter Bernard Ladkin, Bev Littlewood, Harold Thimbleby and Martyn Thomas CBE, ‘The Law Commission presumption concerning the dependability of computer evidence’, 17 Digital Evidence and Electronic Signature Law Review (2020) 1,

[18] Michael Jackson, ‘An approach to the judicial evaluation of evidence from computers and computer systems’ 18 Digital Evidence and Electronic Signature Law Review (2021) 50,

[19] The Royal Society, Royal Academy of Engineering and Royal Society of Edinburgh are collaborating on a ‘Science and the law primer on electronic evidence’. Those preparing the paper include Professors Martyn Thomas CBE, Ross Anderson, Jon Crowcroft and Steven Murdoch. Mr Justice Peter Fraser is also a member. They aim to produce a draft within 12 months, ideally by the end of 2022.


[21] Denise H Wong, ‘Educating for the future: teaching evidence in the technological age’ (2013) 10 Digital Evidence and Electronic Signature Law Review 16; Deveral Capps, ‘Fitting a quart into a pint pot: the legal curriculum and meeting the requirements of practice’ (2013) 10 Digital Evidence and Electronic Signature Law Review 23, (the 2013 edition included a free syllabus),; see also the editorials for 2007, 2010, 2012, 2013, 2016 and 2020.

[22] Scrutinising the contradiction in the claim at the beginning of this article reveals numerous Harbour Cafés up and down the country and none in Lynmouth. The electronic alibi provocatively raised in the opening comment is flawed, as the quote itself suggests. How much other electronic evidence would turn out to be flawed if competently checked? With thanks to James Christie for agreeing to include this example.

By Stephen Mason and Daniel Seng, editors, Electronic Evidence and Electronic Signatures (5th edition, Institute of Advanced Legal Studies for the SAS Humanities Digital Library, School of Advanced Study, University of London, 2021)

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