Scotland Yard secretly reopened an inquiry into the death of former Napier teacher, Blair Peach, 33, two decades after its officers were widely believed to have killed the anti-fascist campaigner.

Relatives of Mr Peach - whose skull was crushed at a demonstration in Southall in 1979 - told the Guardian newspaper they had no idea the file into his death was "reviewed" a decade ago.

No officers were ever charged in connection with the death, although Commander John Cass, who investigated the case as the head of the Metropolian police force's complaints bureau, was believed to have identified several officers as possible suspects in the killing and accused others of thwarting his inquiries.

Police said it would release the Cass report before the end of the year, bringing to an end a 30-year-old mystery.

But the Guardian today reported that a second Met commander in charge of the force's complaints bureau, Ian Quinn, re-opened the Peach file about 10 years ago for a "review", and produced a separate report on the death.

In a statement, the Met said Mr Quinn's decision to reopen the file was taken "following correspondence with various parties, including those representing the family of Blair Peach, around the 20th anniversary of his death".

But the existence of Mr Quinn's report has shocked family and friends of Mr Peach, who had campaigned for more than three decades for full disclosure of all documents relating to his death.

"I would certainly have expected to have been told that there was a review of the case," said Mr Peach's brother, Philip.

"But I was not. It's quite bizarre. The Met have kept this completely secret."

Another brother in New Zealand, Roy Peach, recently said that even if the original report was published in its entirety, the family would never know exactly what happened.

He was allowed access to a part of the report a few years ago but said the family would never be sure of its accuracy because of allegations between police officers.

Mr Peach's partner of 10 years, Celia Stubbs, said she too was shocked to learn that the Met had quietly revisited the case at the end of the 1990s.

"The fact that a commander believed it necessary to look again at Blair's death begs the question, was there more evidence that we did not know about?" she said.

"This means a new layer of confusion and adds to the case for full and open disclosure of everything kept by the Met on Blair's death."

The death of Peach on April 23, 1979 came when Metropolitan police officers were accused of lashing out at protesters opposed to attempts by the National Front to hold a meeting at the heart of the largely Sikh community in Southall, west London.

Forensic pathologists said Mr Peach's head injuries were unlikely to have been caused by a baton, but rather appeared to have been the result of a blow from an unauthorised weapon such as a lead-weighted rubber cosh, a hosepipe filled with lead shot, or possibly a police radio.

In the months after his death the spotlight fell on the Special Patrol Group (SPG), an elite group of officers trained to deal with riots.

When Mr Cass raided SPG headquarters, he uncovered a stash of weapons including a metal cosh - but not the weapon that killed Peach.