The British government has promised that two existing probes would investigate claims that undercover police spied on

the family of the victim of Britain's most notorious racist murder in a bid to smear them.

Home Secretary Theresa May also told parliament on Monday there should be a "ruthless" purge of corruption from police ranks.

The Guardian newspaper reported that a former undercover officer had told them he was part of an operation to spy on relatives of teenager Stephen Lawrence, who was stabbed to death at a London bus stop on April 22, 1993.

Campaigners who were helping the family to push for a wider investigation were also targeted, the report said.

Stephen's father Neville dismissed as "completely unsatisfactory" May's announcement that two ongoing probes - one into police corruption and one into undercover police operations in the 1980s and 1990s - would look into the allegations.

'No confidence'

He said: "I am convinced that nothing short of a judge-led public inquiry will suffice and I have no confidence that the measures announced today will get to the bottom of this matter."

Prime Minister David Cameron said he was "deeply concerned" by the claims, while the commissioner of London's Metropolitan Police, Bernard Hogan-Howe, said he was "personally shocked" by the allegations.

Hogan-Howe, who was not in charge of the force at the time, said: "If these allegations are true, it's a disgrace, and the Metropolitan Police Service will apologise.

"It's imperative that we find out the truth about what happened as quickly as possible."

Insitutional racism

An official report found that "institutional racism" had tainted the original investigation of the 18-year-old's murder, leading to a review and overhaul of police practices.

But in the new claims, the Guardian quoted undercover cop Peter Francis as saying that he had posed as an anti-racist activist in order to try and obtain information which would discredit the campaign for a more thorough investigation.

"I had to get any information on what was happening in the Stephen Lawrence campaign," Francis told the newspaper.

"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."

Two white men, Gary Dobson and David Norris, were convicted of the murder in January 2012 on the basis of new forensic evidence. They lost an appeal against life sentences in August.

They were among five suspects arrested within days of Lawrence's murder, and police say that the investigation into possible accomplices remains "live".