A former undercover police officer says he was ordered to infiltrate the Stephen Lawrence campaign in 1993, the Guardian has reported.
Peter Francis told the newspaper and Channel 4's Dispatches programme he posed as an anti-racism campaigner in a hunt for "disinformation" to use against those criticising the police.
The aim was to smear the dead teenager's family, he said.
Scotland Yard refused to confirm or deny the account.
But a spokesman said the Metropolitan Police shared the Lawrence family's concerns.
Stephen's mother Doreen said the revelation had topped everything she had heard since her son's murder, and she is shocked and angry.
Mr Francis served in the Met's now-disbanded Special Demonstration Squad, which specialised in gathering intelligence on political activists.
In 1993 he says he was ordered to prepare a cover story which would portray him as an anarchist, but when Stephen was murdered and the Lawrence family began a high-profile campaign for justice, the plan changed. He was to pose as an anti-racism campaigner.
He says the Metropolitan Police were concerned the reaction to the Lawrence murder might result in rioting similar to that following the beating of Rodney King in Los Angeles.
He joined the group Youth Against Racism in Europe which rallied around the Lawrence family as they tried to push for Stephen's killers to be found after a botched police investigation.
He told the Guardian and Dispatches his senior officers put him under pressure for "any intelligence that could have smeared the campaign. Along the lines of: the family were political activists, someone in the family was involved in demonstrations, drug dealers, anything. What they would have done with the intelligence, I can't call it, but that was our remit."
At one point, Mr Francis says he was asked to go through a list of visitors to the Lawrence house to provide information about those in contact with the family.
He also claims to have told Scotland Yard that Stephen's friend, Duwayne Brooks, had been involved in violence during a demonstration against the BNP in Welling.
Mr Brooks was traumatised by his experience of watching Stephen die but following Mr Francis's disclosure, police searched CCTV pictures of the protest and he was arrested. The case was eventually dismissed by a judge.
Mr Francis, who used the name Peter Black while under cover, says the aim of his operation was to ensure that the public "did not have as much sympathy for the Stephen Lawrence campaign" and to persuade "the media to start maybe tarring the campaign".
Asked what information he produced, he said: "I wasn't successful, no SDS officer was successful, in finding anything really concrete. It was really just a bit of hearsay, tittle-tattle."
Doreen Lawrence said she was shocked and angry at the disclosure.
"It just makes me really, really angry," she said, "that all of this has been going on and all the time trying to undermine us as a family.
"Somebody sitting somewhere, calculating what, you know, what they'd be doing to look at and infiltrate, our family. It's like, we're treated as if to say we're not human beings.
"We weren't linked to any political groups, you know, we weren't linked to any of them so at the time.
"Nothing can justify the whole thing about trying to discredit the family and people round us."
The Metropolitan Police would not confirm or deny the account given by Mr Francis, but admitted "the claims in relation to Stephen Lawrence's family will bring particular upset to them and we share their concerns".
An independent investigation into a number of allegations against former under cover police officers, codenamed Operation Herne is under way.
In a statement the Met said: "Any actions by officers working on or with the Special Demonstration Squad need to be understood by Operation Herne in terms of the leadership, supervision, support, training, legal framework, tasking and reporting mechanisms that were in place at the time."
But the force gave the same response to allegations that another undercover officer had helped write the leaflets at the centre of the McLibel trial in the mid 1980s.
The statement said: "At some point it will fall upon this generation of police leaders to account for the activities of our predecessors, but for the moment we must focus on getting to the truth."
Shadow home secretary Yvette Cooper said: "These are shocking and appalling allegations and we need to know the full truth of what happened.
"An investigation is currently under way into the actions of undercover police officers within the Met.
"However, given the significance of the Lawrence case, and the unresolved concerns about corruption too, the Home Secretary should seek a faster investigation into these specific allegations," she said.
Mr Francis told the Guardian he had come forward because of the "morally reprehensible" way in which under cover officers had sometimes worked.
He is particularly angry his role was never discussed by the Stephen Lawrence public inquiry chaired by Sir William Macpherson. He claims senior officers deliberately chose to withhold the information from the inquiry.
Stephen Lawrence, a black teenager, was killed as he waited for a bus in April 1993.
More than 18 years later, in January 2012, Gary Dobson and David Norris were found guilty of his murder by an Old Bailey jury after a review of the forensic evidence.
Dobson and Norris had first been arrested in connection with the murder just weeks after it happened, but the case against them collapsed.
In 1999, an inquiry chaired by Sir William Macpherson - a retired High Court judge - into the killing and its aftermath published a report accusing the police of institutional racism.
Criticising the police, Sir William said he was "astonished by the lack of direction and organisation during the vital hours after the murder".
The Lawrences had been patronised, treated with "insensitivity and lack of sympathy", and kept in the dark about the investigation, he added.
Dispatches is broadcast on Channel 4 on Monday 24 June at 20:00 BST.