The Prime Minister has said he is "deeply concerned" at claims the police attempted to 'smear' the family of murdered teenager Stephen Lawrence.
David Cameron also demanded an immediate investigation into the Metropolitan Police following the allegations by former undercover police officer Peter Francis.
Francis told the Guardian that the Metropolitan Police wanted him to find "dirt" on the family of Lawrence, shortly after he was murdered in a racist attack in 1993.
The undercover officer claimed he was asked to target people launching a campaign against the force, who were angry at their failure to bring Lawrence's murderers to justice. He also claims he was asked to target Duwayne Brooks, one of Lawrence's friends who witnessed the murder.
Posing as an anti-racism activist in the mid-1990s, Francis claimed he was under "huge and constant pressure" to find information on the family, which would ultimately weaken the campaign against the police.
He added: "I had 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.
"Throughout my deployment there was almost constant pressure on me personally to find out anything I could that would discredit these campaigns".
After news of the claims broke, Cameron tweeted: "I'm deeply concerned by reports that police wanted to smear the family of Stephen Lawrence. The Met must investigate immediately".
Responding to Francis's claims, Doreen Lawrence, the mother of the murdered teenager, described her anger at the allegations. She told the Guardian: "Out of all the things I've found out over the years, this certainly has topped it.
"It just makes me really, really angry 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.
"Nothing can justify the whole thing about trying to discredit the family and people round us."
Scotland Yard says it understands the seriousness of the allegations and has already launched an investigation into the matter.
A Met Police spokesperson said: "The claims in relation to Stephen Lawrence's family will bring particular upset to them and we share their concerns.
"A thorough review and investigation into these matters - Operation Herne - is being overseen by Derbyshire Chief Constable Mick Creedon.
"Operation Herne is a live investigation, four strands of which are being supervised by the Independent Police Complaints Commission, and it would be inappropriate to pre-judge its findings."
Shadow Home Secretary Yvette Cooper, called for Theresa May to seek a faster investigation into the allegations.
She said: "These are very serious new claims about the conduct of the police at the time, and it is vital we get to the truth about what happened.
"Victims need to be able to have full confidence in the vital work the police do each day to keep them safe and get them justice. That is why any suggestion that undercover policing could have undermined victims or justice should be taken so seriously and the truth pursued now".
Jack Straw, the former home secretary, said he is "profoundly shocked" by the claims and is considering referring the case to the Independent Police Complaints Commission (IPCC).
Scotland Yard has so far refused to confirm or deny the claims by Francis.
The force added: "The Met must balance the genuine public interest in these matters with its duty to protect officers and former officers who have been deployed undercover, often in difficult and dangerous circumstances.
"We are therefore not prepared to confirm or deny the identity of individuals alleged in the media to have been working undercover, nor confirm nor deny the deployment of individuals on specific operations."
"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."
In 1999, a report by the Macpherson inquiry into the killing and the police's investigation accusing the Met police of "institutional racism".