One of Britain's leading human rights lawyers has demanded a fresh police inquiry to establish what the British intelligence services knew about the murder of a prominent anti-nuclear campaigner.

Michael Mansfield QC said new evidence meant that an independent police force should be appointed to examine enduring concerns and inconsistencies relating to the death of Hilda Murrell in March 1984.

Murrell, 78, was abducted from her home in Shrewsbury and her body was discovered days later in a nearby copse. A high-profile campaigner against nuclear weapons, she had been due to present evidence to the public inquiry into the proposed Sizewell B nuclear reactor in East Anglia. Her death triggered numerous conspiracy theories and allegations relating to the involvement of MI5, with one MP, Tam Dalyell, telling parliament that "men of British intelligence" were involved.

Subsequent claims from intelligence sources that they never even opened a file on the rose-growing anti-nuclear campaigner have now been dismissed by Mansfield as "completely ludicrous".

He said: "There must have been a file for a number of reasons. One of them being that she plainly was very active and very outspoken about a government policy that was extremely sensitive at that time – nuclear power.

"It was central to Margaret Thatcher's thinking. They would have been watching closely what she was up to, who she was associating with and so on.

"The victim was consumed with anxiety that something was going to happen to her. A look at why that might be involves the evidence she was about to give to the Sizewell inquiry."

The involvement of Mansfield, whose past cases include the Stephen Lawrence murder, follows the painstaking accumulation of evidence on the case by Murrell's nephew, Commander Robert Green.

The former naval intelligence officer was one of a handful of people privy to details of the sinking of the Argentinian ship the General Belgrano, during the 1982 Falklands conflict. Green became embroiled in allegations that he leaked intelligence to Dalyell that the Belgrano had been attacked while steaming away from the Falklands, a revelation that undermined Britain's justification for the sinking.

Just two days before Murrell was abducted, Dalyell began asking ministers detailed questions about the movements of the Belgrano when it was sunk.

Murrell's links to Green and her outspoken nature may have placed her in the spotlight of the intelligence agencies. "They [the security services] must have noticed his connection with her. Therefore they might have thought that she possessed information of a sensitive nature," said Mansfield.

Despite 28 years having passed since her death, Green will this week reveal details of what he claims are attempts to intimidate him in order to prevent him from investigating the case. Despite having moved to New Zealand, Green says he is the subject of continuing surveillance and that the tyres of his car have been slashed, his mail intercepted and, occasionally, his house broken into.

He has continued to investigate, arriving in London this week to share fresh evidence collated for his book on the murder, A Thorn In Their Side.

Among questions raised about the case are those casting fresh doubts on the conviction of a burglar, Andrew George, who was jailed for life in 2005 for Murrell's murder. George was aged 16 at the time and in care at a children's home near her home. The prosecution believed that he panicked during a burglary before abducting Murrell.

George's DNA was found to match samples taken from the scene, yet a previously undisclosed witness statement made by a forensic scientist in the case, Michael Appleby, indicates that he found DNA under Murrell's fingernails from another man.

Green claims that this information was withheld from the trial jury.

Another troubling aspect of the case relates to the testimony of the owner of the copse where the body was discovered. On the day after the murder, Captain Ian Scott visited the copse to check for trees that needed felling. Despite visiting the exact spot where her body was found, Scott somehow missed it. Yet photographs clearly show the body being visible from a distance. Subsequent police inquiries suggested that he was looking up at trees and would not have been studying the ground.

"There was no way that somebody of his calibre, of his knowledge, would have overlooked Hilda's body," said Mansfield. "Together, these factors require a reinvestigation in relation to material that never really surfaced in any of the judicial proceedings, either the inquest or the trial itself."