Charles and Diana Ingram will ask the court of appeal to overturn their convictions for cheating on Who Wants to Be a Millionaire? – arguing that new audio analysis casts serious doubt on the evidence used to prosecute them.

The human rights lawyer Rhona Friedman told the Guardian she had agreed to take on the case after becoming convinced the couple were the victims of a miscarriage of justice. She intends to allege that audio evidence may have been altered before it was presented to the jury.

“Speaking to them and meeting them, I just know that they did not do it,” said Friedman, confirming the paperwork associated with the case will be filed this summer with hopes for a hearing at the end of the year. “I just feel very sorry for them.”

The couple, portrayed on ITV’s hit mini-series Quiz which aired this week, achieved infamy in 2003 after they were found guilty of conning their way to win £1m on what was one of the country’s most popular gameshows.

Tecwen Whittock arriving at Southwark crown court in London in March 2003. Photograph: Johnny Green/PA

The programme’s producers were immediately suspicious of how the serving army major won the prize and reported him to the police. The prosecution successfully argued that they “procured the execution of a valuable security by deception” by colluding with Welsh lecturer Tecwen Whittock, who coughed at crucial moments to guide Charles Ingram to the enormous cash prize.

Friedman described Ingram as an intense, emotional man whose “wild and eccentric” character could have been mistaken by the show’s producers for suspicious behaviour: “The performance he gave that everyone found remarkable is pretty much how I experienced Charles.”

Friedman, co-founder of Commons, the not-for-profit criminal law firm, took on the case after being approached by the late investigative journalist Bob Woffinden, who dedicated his life to overturning miscarriages of justice.

She said modern improvements in audio analysis mean it was possible to identify individual coughs from the audience, which cast doubt on the idea Whittock’s wheezing helped win the competition – and encouraged fans of the TV show to sign up to a new site for those following the case.

The couple’s appeal will also claim that audio on the recording played to the jury was edited, giving the “opportunity to question the integrity of the evidence”.

She said: “There are some really weird anomalies. There are some glaring omissions. You’ve got different microphones picking up different people. There are inconsistencies between what is on one tape and another.

“The idea is that Charles was primed to appear on TV for the first time ever, in front of millions of people, knowing that he was going to be cheating, with a guy sitting behind him that he hasn’t met coughing to indicate an answer, without giving any reaction. It would have been the performance of a lifetime and he’s not capable of it.”

Contrary to his public image as an aloof former British Army officer, Charles Ingram now maintains a vocal Twitter account which he has used to campaign against Brexit, criticise the government’s response to the coronavirus pandemic, and talked about his hope that Keir Starmer can revive the fortunes of the Labour party.

In recent days he has shared his reminiscences about the show, saying that Diana has taken to calling him “Matthew” in reference to actor Matthew Macfadyen who plays him in Quiz. The couple now live near Bath where Diana runs a jewellery business.

From left: Sian Clifford as Diana Ingram, Matthew Macfadyen as Charles Ingram and Michael Sheen as Chris Tarrant in ITV drama Quiz. Photograph: Matt_Frost/ITV

Quiz, by the acclaimed writer James Graham, began as a hit theatre production, which allowed the audience to vote at the end of each half on whether they thought the Ingrams were guilty. Almost every audience initially concluded they were, before changing their minds after hearing the defence case.

The production team said the three-part television adaptation was shown across consecutive nights in order to become ‘event television’ – the same justification used for the original series of Who Wants to Be a Millionaire?.

Graham previously wrote Channel 4’s Brexit: An Uncivil War, which helped elevate political campaigner Dominic Cummings in the public consciousness. He is currently working on a film adaptation of Ink, his play about Rupert Murdoch and the founding of the Sun newspaper.