The stabbing of 18-year-old Stephen Lawrence in 1993 shocked the UK and came to be seen as a festering racial injustice. It took 19 years before anyone was convicted and the case exposed racism and incompetence in Scotland Yard.
In an interview with The Guardian newspaper, former officer Peter Francis says he was charged by his superiors to find anything that could used against Lawrence's parents and family.
He was also asked to discredit Stephen's close friend, Duwayne Brooks, who witnessed the stabbing, and to find information that could undermine campaigners who were angry with the police failure to bring the killers to justice.
Mr Francis said his superiors "wanted to get any information on what was happening in the Stephen Lawrence campaign. They wanted the campaign to stop. It was felt it was going to turn into an elephant."
Mr Francis, who posed as an anti-racism activist, said they withheld his role from Sir William Macpherson, who led a public inquiry into the bungled police investigation.
He said senior officers thought anger over Stephen's killing would trigger riots in London. They had "visions of Rodney King", he said. Mr King's beating by police led to the 1992 LA riots.
Stephen's mother, Doreen, said Mr Francis's revelations were surprising. "Out of all the things I've found out over the years, this certainly has topped it.
"Nothing can justify the whole thing about trying to discredit the family and people around us."
Last year Gary Dobson, 36, and David Norris, 35, were convicted of killing Stephen.