There are 'similarities' between Mr Peach and Mr Tomlinson's deaths
A campaign group has called on the Metropolitan Police Commissioner Sir Paul Stephenson to publish a report into the death of a protester in 1979.
Blair Peach, 33, a teacher, died from a head injury as police were dispersing protesters during an anti-fascist protest in Southall, west London.
Inquest said publishing the report would be "timely" after the death of Ian Tomlinson during the G20 protests.
The force said it "cannot act in a manner that could cause distress".
Mr Tomlinson, 47, a newspaper vendor, died on 1 April after being apparently pushed over by an officer during the G20 protests in London in April.
In a letter Inquest, which supports the families of people who die in custody, called on Sir Paul to publish the report on the 30th anniversary of Mr Peach's funeral.
The report was written by a then police commander John Cass.
Deborah Coles, the co-director of Inquest, said: "We are supporting the families involved, and others, who have been trying to get the Cass report disclosed for nigh on 30 years, and, given the public disquiet about the death of Ian Tomlinson, and the similarities between the two deaths, we felt disclosure of the report would be very timely."
A police spokesman said: "We said in May that the Metropolitan Police has a duty of care to the family, and cannot act in a manner that could cause distress, where there is either a legal restriction or a reasonable justification for not doing so."
Mr Peach, from New Zealand, was member of the Socialist Workers' Party and was fatally injured during the protest on 23 April 1979.
At the time eleven witnesses claimed to have seen members of the Metropolitan Police Special Patrol Group (SPG) hitting Mr Peach in a side-street at the height of the violence. No officers were charged with the alleged attack.
An inquest recorded a verdict of death by misadventure, and the Met reached an out-of-court settlement with Mr Peach's brother in 1989.
Current Justice Secretary Jack Straw was a backbencher when he and 79 MPs called for a public inquiry into the case after the inquest, but the request was rejected by the government.