Robert Green suspects MI5 involvement in the death of his aunt 30 years ago.
On Saturday night he spoke over the internet from his home in New Zealand at a public meeting in Shrewsbury.
He told the audience of about 50 people at Shrewsbury's Morris Hall that he is convinced information is being withheld about the death of his aunt.
Miss Murrell, 78, was abducted from her Shrewsbury home on March 21, 1984. Her body was found near Haughmond Hill three days later.
She had been repeatedly stabbed and left to die of hyperthermia.
Twenty-one years later labourer Andrew George was convicted of the killing. But Mr Green believes the wrong man is in prison – and wants a Hillsborough-style inquiry to find the real truth.
Mr Green said Miss Murrell was a vehement anti-nuclear campaigner who carried out work to expose the perceived risks at the planned Sizewell nuclear reactor in Suffolk.
She was also suspected of having evidence relating to the sinking of the Argentine cruiser General Belgrano in the Falklands War and she also appears to have had links with the World War II code-breaking headquarters in Bletchley Park.
Speaking via video link from his home in Christchurch, Mr Green said: "There is unwavering evidence that British intelligence was involved in the murder. She obtained copies of sensitive documents and this might be related to the nuclear industry or the Falklands War.
"Andrew George was encouraged to burgle Hilda's house and then was used a preposterous fall guy. There is an urgent need to reopen the case and release Andrew George who simply couldn't have abducted and killed Hilda."
Also speaking at the meeting was one of Miss Murrell's friends, Trina Child, who claimed to have spoken to a prisoner who had compelling information about the murder.
Miss Child said the man had spoken to another prisoner who was one of four sent by the Secret Service to interrogate Miss Murrell over the information she had.
She said: "He gave a very detailed description of what happened in the house. This was an unofficial group of thugs who went around and did the Government's dirty business."