The legal team acting for the sister of Ruth Ellis, the last woman to be hanged in Britain, believe the papers relate to a meeting that her solicitor had with Scotland Yard in 1973 - 18 years after she was hanged.
One theory is that the solicitor, John Bickford, had known all along about the involvement of another man in the murder and that in 1973, when he was a dying man, he had gone to tell the police what had really happened.
"The closed file we have found at the Public Record Office has a 30- year closure order on it, dating from 1973. She was executed in 1955, and as far as we are aware, nothing else happened in 1973 that could have prompted such a secrecy order," said one of the lawyers, Lynne de Maid, a member of the Cardiff-based legal team that successfully won a posthumous pardon for Mahmood Mattan, the executed Cardiff seaman.
Ruth Ellis was executed for the murder of her lover, David Blakey, whom she shot. The trial excited worldwide interest and has since generated more than 30 books.
Ms de Maid said: "At the time she was going out with Blakey, she also had another boyfriend, Desmond Cousens.
"Ruth left home at 15, was conned by a Canadian airman and became pregnant by him.
"She met George Ellis who was very violent, and then Blakey who was physically very violent and who punched her in the stomach, causing a miscarriage three weeks before the shooting.
"We know that Ruth fired the gun, but we think it was a joint enterprise; that Cousens borrowed a gun, cleaned it, gave Ruth target practice, plied her with Pernod and then drove her to the scene of the crime.
"We think Bickford knew that, but listened to Cousens perjuring himself in court when he should have done something about it. Just why he did that, we do not know yet.
"In 1973 Bickford went to Scotland Yard, where we believe he confessed that he didn't represent her properly. Ruth did not expect to hang and we think she was encouraged in that belief by her solicitor.
"It was only on the day before she hanged, when she sacked him, that she realised what was happening," Ms de Maid said.
"We have made repeated requests for access to the closed file and we have now written to ministers seeking their help."
The team expects to submit an application for an appeal with the Criminal Cases Review Commission by the end of this month, which will include the details of the attack by Blakey.
"We feel very strongly that they will refer it, and we think the case will be heard next year at the Court of Appeal. She was not given a fair hearing," Ms de Maid said.
"When you read the papers on this case and that of Mattan, you realise the appalling standards of justice that operated in those days. Facts about her miscarriage were not even brought out at the trial. It is very sad to think of this young woman with a two-year-old and a 10-year-old going to her death."
The team is also putting together a file on her psychological condition after her miscarriage, including depression and jealousy, both of which might now be mitigating factors, reducing the conviction from murder to manslaughter.Reuse content