Blair Peach, 33, was killed after he was hit over the head by an officer at an anti National Front demonstration in Southall, west London, in 1979.
Police officers involved made "false statements" during the investigation into his death, a previously secret report found.
Met police chief Sir Paul Stephenson said the report made "uncomfortable reading" but despite this the force insisted that nobody involved would face prosecution.
Mr Peach's partner Celia Stubbs said the findings "vindicated" her belief that the activist was killed by police.
The document – written by Commander John Cass, a former senior officer at the Met's internal complaints department, singled out the actions of the three officers, named as E, H and F.
He wrote: "The most serious aspect of this case has, without doubt, been the obstruction of the investigating officer in execution of their duty."
Commander Cass recommended the three officers should face prosecution.
He added: "I strongly recommend that proceedings be taken against Officer E, Officer H and Officer F for obstructing police in the execution of their duty, conspiring to do so, and attempting or conspiracy to pervert the course of justice."
Mr Peach was struck on the head during the protest on April 23 1979. He died in hospital of head injuries the next day.
Some 14 witnesses reported seeing him being hit by a police officer.
The names of the officers and witnesses involved were removed from the report for legal reasons.
Police insisted this had not been done to prevent embarrassment to the force.
The report said it could "reasonably be concluded that a police officer struck the fatal blow".
It continued: "The attitude and untruthfulness of some of the officers involved is a contributory factor.
"It is understandable that because of the events of the day officers were confused, or made mistakes, but one would expect better recall of events by trained police officers.
"However, there are cases where the evidence shows that certain officers have clearly not told the truth."
Mr Peach's partner Celia Stubbs said she was delighted to finally see the report.
She said: "I never really believed we would get it.
"It is fantastic after 31 years. I have only read 200 pages of the report but I feel that we have really been vindicated because we have always said that Blair had been killed by a policeman.
"It says in the report that it was an officer that struck Blair.
"I never really expected a prosecution. I don't regret that, I am just pleased that we have the report so we can see what happened on the day."