Prime Minister David Cameron called on Monday for an investigation into claims that police spied on the family of a black teenager whose murder 20 years ago exposed what an inquiry called "institutional racism" in London's police force.
In one of Britain's highest-profile racial killings, Stephen Lawrence was stabbed to death at a bus stop in southeast London in 1993. An inquest ruled it had been an unprovoked attack by a gang of white youths.
Not until last year was anyone convicted for the murder.
Now a former undercover police office has said he was asked to infiltrate and discredit the Lawrence family's campaign for justice in the years following the murder.
Cameron's spokesman said in a statement: "The Prime Minister is deeply concerned by reports that the police wanted to smear Stephen Lawrence's family and would like the Metropolitan police to investigate immediately."
In the Guardian newspaper, former undercover officer Peter Francis said he spent four years posing as an anti-racist campaigner after his superiors tasked him with trying to uncover information that could be used against the 18-year old student's parents.
Francis said senior officers were afraid that anger at the failure to launch a more thorough investigation into the murder would spiral into disorder on the streets.
He monitored several campaigns, involving relatives of mostly black men who had died in suspicious circumstances in police custody.
He never met the Lawrence family but said he passed back "heresay" about them to his superiors.
He also said senior officers witheld his undercover role from Sir William Macpherson, who headed a public inquiry to examine the police investigation into the death.
Macpherson said in a damning 1999 report that that investigation had been "marred by a combination of professional incompetence, institutional racism and a failure of leadership by senior officers."
The case still weighs on London's police, despite an overhaul of policies and an attempt to recruit more officers from ethnic minorities.
Two inquiries are already under way: one is by Derbyshire Chief Constable Mick Creedon into allegations that undercover officers in other operations assumed the the identities of dead babies and had inappropriate sexual relationships with the subjects of their surveillance.
The other, by barrister Nick Ellison, is looking at claims police corruption hindered the original murder Lawrence murder investigation.
London's Metropolitan Police noted in a statement after the latest revelations that the Creeedon inquiry was already under way.
"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," it added.
Home Secretary Theresa May said in parliament later on Monday that the two inquiries would work together to examine the latest claims.
Labour is demanding a single, independent inquiry focused just on the Francis claims of discrediting the family.
Those claims follow a joint investigation between the Guardian and Channel 4's Dispatches programme. A related programme was due to be broadcast by C4 in Britain on Monday evening.