An expert witness was asked by a Post Office prosecutor to consider changing his testimony to avoid a "damaging concession", the OceanNewsUK has found.
Fujitsu engineer Gareth Jenkins rephrased parts of a report on the Horizon IT system after advice from barrister Warwick Tatford.
The evidence was used in the 2010 case against postmistress Seema Misra, who was wrongly jailed while pregnant.
The Post Office has said it is "focused on righting the wrongs of the past".
Gareth Jenkins, former chief IT architect at Fujitsu, is one of only two people involved in the Post Office scandal facing possible legal consequences, as the Metropolitan Police investigate him for potential perjury.
But the full versions of Mr Jenkins' draft report - obtained by the OceanNewsUK - raise new questions about his 2010 testimony as an expert witness and the potential influence of a Post Office lawyer on him.
Expert witnesses produce reports for use in court that must be independent and unbiased, regardless of who the witnesses work for, according to Criminal Procedural Rules, which set out how courts in England and Wales operate. Lawyers should ensure that experts understand all this.
Mr Jenkins' testimony was used in the case against Seema Misra, who served four months in prison. She was one of more than 900 sub-postmasters who were wrongly prosecuted between 1999 and 2015 because of Horizon - a faulty accounts computer system which showed errors that did not exist.
The changes to Mr Jenkins' expert report were not disclosed to the defence. Mrs Misra, whose conviction was only quashed in 2021, said knowing the expert witness testimony had been altered "could have saved me, or it could have turned the case the other way round".
Legal experts say Mrs Misra's trial was important for the Post Office to demonstrate that there were no problems with the Horizon system. "It was one of the few known challenges to Horizon in a criminal trial," said Prof Richard Moorhead, an expert in law and ethics at the University of Exeter.
Draft versions of Gareth Jenkins' report show that barrister Warwick Tatford advised him to consider taking a stronger position on certain technical issues raised by Prof Charles McLachlan, an expert witness for Mrs Misra's defence team.
In one draft statement - dated 6 October 2010 - Mr Jenkins said that he could not "10% [sic] rule out" problems with Horizon screens as a possible cause for some shortfalls. During the ongoing public inquiry into the Post Office scandal, it was agreed that Mr Jenkins meant to say "100%".
Mr Tatford wrote in response: "Please rephrase. This will be taken as a damaging concession."
In his final written testimony, Mr Jenkins stated that "no scenario has been presented" that could explain losses because of poorly calibrated touch screens.
Elsewhere in the draft report, Mr Jenkins noted "there doesn't appear to be a thorough justification" for a list of "hypothetical" Horizon issues raised by the defence team's IT expert.
Mr Tatford asked if he could use "stronger wording", saying: "There doesn't appear to be any evidential bases for the hypotheses at all."
In his final report, Mr Jenkins added a line to say: "I can see no evidence to support these hypotheses."
Mr Jenkins also initially seemed to agree with a theory made by the defence expert about corrections to missing transactions.
Mr Tatford wrote back: "Why?" The barrister added there was "no evidential basis whatsoever" for the defence expert's assertion.
In the final version of the report, Mr Jenkins changed his opinion, saying there did not "appear to be any evidence" to support the theory made by the defence expert.
In another example, Mr Jenkins agreed with the defence expert that sub-postmaster training issues could account for some shortfalls, stating: "I support his finding regarding discrepancies in cash in almost every period."
Mr Tatford responded saying that his agreement "might be interpreted as a concession" that the Post Office's case was "entirely flawed".
In reality, said Mr Tatford, "discrepancies are always to be expected", but sub-postmasters would be expected to discover the source of significant errors and correct them.
In his final statement, Mr Jenkins toned down his agreement, and said cash discrepancies indicated "at least poor management within the branch, and probably something more serious".
However, Mr Jenkins pushed back at some suggestions by Mr Tatford, pointing out that his expertise was in how Horizon worked, not how post offices operated.
The Post Office has maintained that they advised Mr Jenkins appropriately at the time but the surfacing of these documents "catches them red-handed", according to the University of Exeter's Prof Richard Moorhead.
He added: "What you see is Warwick Tatford trying to influence the evidence of the expert witness and you see Mr Jenkins sometimes resisting and sometimes conceding changes."
Prof Moorhead said he believed that these documents - along with other evidence produced by the Post Office inquiry - were enough for the police to investigate Mr Tatford and others in the legal team.
When questioned at the Post Office inquiry last November about some of the changes to Mr Jenkins' evidence, Mr Tatford apologised for mistakes in his approach.
He said he had "overstepped the mark" but denied he had "materially" altered Mr Jenkins' testimony.
In a statement to the OceanNewsUK, Warwick Tatford said that he answered all questions put to him "in good faith" at the inquiry and deemed it "inappropriate to comment further" while the inquiry was continuing.
He added that he had given "an unreserved apology" to Seema Misra at the inquiry and he "was sorry [she] had not received a fair trial".
Seema Misra said she accepted Mr Tatford's apology.
"He's the first person, and only person from the Post Office side, to apologise directly to me," she said.
But seeing the full drafts for the first time, she said it was "horrible" that Mr Tatford seemed to be "putting words into [Mr Jenkins'] mouth".
In addition to Mr Jenkins' final testimony, his draft statements should have been kept by the Post Office and given to the court and Seema Misra's lawyers, as part of its disclosure obligations under the Criminal Procedure and Investigations Act.
Disclosure is where each side is required to make available any material which is relevant to disputed issues in a case.
Mrs Misra's former solicitor, Issy Hogg, said she was "surprised" by Mr Tatford's annotations, which she said showed him trying "to shore up" Gareth Jenkins' testimony.
Although Ms Hogg was unaware of how Mr Tatford had advised the expert witness, she said Mrs Misra's legal team had separately made an "abuse of process" application in February 2010 to try to halt the case. This was because they had concerns about the way Mr Jenkins was chosen as an expert, his independence and how he had been instructed.
She said that "frustrating" disclosure failures throughout Seema Misra's case contributed to the sub-postmistress not having a fair trial.
"Post Office lawyers have a lot to answer for," said Ms Hogg.
In the run-up to the trial she had asked the Post Office legal team for a known-error log - a document made by Fujitsu which recorded errors within Horizon. But this was not disclosed to her team or discussed in Mr Jenkins' expert reports.
Had her team been given the known-error log, Ms Hogg said they would have been able to challenge the Post Office's case that there were no issues with the IT system.
The Bar Standards Board, which regulates barristers, said it did not comment on individual cases. While "some matters" relating to the Post Office scandal had been referred to investigation, it said it was unlikely to take any regulatory action until the inquiry had heard all the relevant evidence.
A lawyer for Gareth Jenkins said it would be "inappropriate" for him to comment ahead of him giving evidence to the Post Office inquiry in June.
