Scotland Yard commissioner Sir Bernard Hogan-Howe has urged a former undercover officer who claimed there were attempts to smear Stephen Lawrence's family to speak to police.
Peter Francis, who worked as a police mole in the Metropolitan Police's Special Demonstration Squad in the 1990s, has so far refused to speak to investigators looking at the activities of undercover officers.
He claimed that undercover teams were told to look for information they could potentially use to smear the Lawrence family as they campaigned for justice for their murdered son.
Additional allegations have been made that police bugged meetings with Mr Lawrence's friend Duwayne Brooks, who was with the 18-year-old on the night he died, and his lawyers.
Sir Bernard said: "One of the critical things is to talk to Peter Francis. We've seen him appear on television, we know he's talked to journalists, it would be really helpful if he talked to the investigators because that will help to get to the bottom of it."
Derbyshire Chief Constable Mick Creedon is running an investigation, Operation Herne, into the activities of undercover police officers, and has prioritised the Lawrence smear claims. In July he told MPs his team had found no evidence to back the allegations.
Appearing before the Home Affairs Select Committee, he said: ''There is nothing in Operation Herne which suggests any attempt whatsoever to do two things: firstly to be tasked against the Stephen Lawrence family, and secondly to besmirch the Stephen Lawrence family.''
Stephen's parents Neville and Doreen have called for a public inquiry into the claims, which they believe is the only way to get to the truth.
Mr Francis has told the Guardian newspaper he will only give evidence to a public inquiry. Police watchdog the Independent Police Complaints Commission said it could not properly assess the claims without speaking to Mr Francis.
IPCC deputy chairwoman Deborah Glass said on July 26: ''I have written to Mr Francis to ask if he would be willing to speak to the IPCC. I have not yet had a response, so can only rely on his statements as reported in the Guardian that he is only willing to assist a public inquiry."