Officials say British Prime Minister David Cameron wants 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 Monday published claims that the officer infiltrated anti-racism protest groups to gain information that could discredit the family of Stephen Lawrence.

Lawrence died following a racist attack in 1993. An inquiry into the way London's Metropolitan Police investigated the murder concluded that the force was "institutionally racist".

Former undercover officer Peter Francis claimed that while posing as an anti-racism activist he was also asked to target Lawrence's friends and other campaigners angry at the failure to bring the killers to justice.