LONDON — British Prime Minister David Cameron called Monday for an immediate investigation into allegations that an undercover police officer was ordered to smear the reputation of the family of a murdered black teenager.

The Guardian newspaper published claims Monday that the officer infiltrated anti-racism protest groups to gain information that could discredit the family of 18-year-old Stephen Lawrence.

"To hear that, potentially, the police that were meant to be helping them were actually undermining them — that's horrific," Cameron said in Downing Street. "We must make sure those investigations get rapidly to the bottom of what's happened and we get the full truth out."

Lawrence was stabbed in 1993 and an ensuing inquiry found the Metropolitan Police "institutionally racist" in its response to the killing. It said the Lawrence family had been treated insensitively and unsympathetically by police officers, who deliberately withheld information and denied the murder was a racist attack.

Last year, almost 20 years after the murder, two men were convicted for the murder following the discovery of new scientific evidence and covert video footage. One had previously been acquitted in 1996. Three other suspects in the case remain free and deny any involvement in Lawrence's death.

In the Guardian, former undercover officer Peter Francis claimed that he came under constant pressure from his superiors to "hunt for disinformation" that might be used to undermine those arguing for a better investigation into Lawrence's murder. At the time — around the mid 1990s — Francis had been posing as an anti-racism activist for groups such as Youth Against Racism in Europe.

The former officer said senior officials feared that anger over the failings of the investigation would spiral into disorder similar to the 1992 riots in Los Angeles.

Francis also claimed that during his four-year undercover spell he attempted to target Lawrence's friends and other anti-racism campaigners.

Home Secretary Theresa May said two existing inquiries into police wrongdoing will investigate the allegations. But Lawrence's father, Neville, called for a judge-led public inquiry in the wake of the allegations, saying he had "no confidence" in the measures announced Monday.

Ex-Scotland Yard chief Paul Condon said he had never authorized or condoned any smear operation against the Lawrence family and was "shocked and dismayed" by the former officer's allegations.

The Lawrence case became a watershed moment for the fight against racism in the U.K., prompting new laws and reforms to public services including the police force.

The MacPherson inquiry made 70 recommendations for reform in Britain, including criminalizing racist statements made in private and a double jeopardy law that allowed retrials following the uncovering of new evidence.