Trina Guthrie, 41, who regarded Miss Murrell, 78, as an aunt, says she has evidence to suggest the murder was carried out by four people hired by the security service, MI5, to steal information about the sinking of the Argentine cruiser General Belgrano during the Falklands war in 1982.
The evidence has come from a prisoner interviewed by Ms Guthrie between November 1991 and February 1992. He told her that he formed a friendship with another prisoner, currently serving 15 years for armed robbery, who claimed to have led the group that carried out the murder.
Ms Guthrie, an official of the Royal Society of Nature Conservation and a prison visitor, swore an affidavit, recently published in Enemies of the State, a book about the security services by Gary Murray.
West Mercia police have now set up a special incident room at Shrewsbury to look into the matter. Chief Insp Peter Herbert said: 'I am attaching great important to the contents of Trina Guthrie's affidavit.'
Ms Guthrie's affidavit does not include the names of the alleged murderers. She said she welcomed the police investigation and was prepared to assist, but would reveal the names of the three people she has been told were involved - the fourth remains a mystery - only to a public inquiry.
'I don't want it to be dismissed as not good enough. I've got to get it into the public arena so that interest is kept up and it is not white-washed,' Ms Guthrie said.
Hilda Murrell's half-naked body was found in March 1984 in a copse near Hunkington, near Shrewsbury. Covered by a coat, she had suffered multiple stab wounds, although a pathologist found she died of hypothermia.
After a nine-month investigation during which 3,600 potential suspects were 'researched', police told an inquest they had reached the 'inescapable conclusion' that Miss Murrell died after disturbing a burglary.
This failed to satisfy the anti- nuclear lobby, which was concerned that her death came shortly before she was due to speak at the public inquiry into the Sizewell B nuclear plant.
A number of MPs, including Labour's Tam Dalyell, believe her death may have been linked to the sinking of the General Belgrano. Miss Murrell's nephew, Commander Robert Green, was a member of the crew of HMS Conqueror, the British submarine which sank the warship.
Last night, Ms Guthrie said she believed Miss Murrell was murdered after her attackers failed to find the sought-after documents. She had been told that while two people drove to Miss Murrell's Welsh cottage hoping to find the documents, two others decided to intimidate Miss Murrell herself in the hope that she might reveal where the papers were.
'They found a citrus fruit - either an orange or a grapefruit - and after binding Hilda in a sheet, they stabbed the fruit with a knife. They told Hilda that unless she told them where the documents were stored she would suffer the same injuries. Hilda's fear must have excited them. One slashed her upper arm,' Ms Guthrie said.
A Home Office patholgist who carried out a post-mortem confirmed the stabbing detail at the inquest.
Later Miss Murrell was taken to an old army base outside Shrewsbury to be questioned further. At that point, with hospital treatment she might have survived, the pathologist told the inquest. Instead she was dumped while still alive.