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 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.
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."
But Stephen's mother Doreen Lawrence has backed Mr Francis's decision only to speak to a public inquiry, which she and the teenager's father Neville see as the only way of getting to the truth.
Her lawyer Imran Khan said: "He should be giving evidence to a public inquiry not staying behind closed doors and speaking to police officers. That's the whole point of calling for an inquiry."
Derbyshire Chief Constable Mick Creedon is running an investigation, Operation Herne, into the activities of undercover police officers, and says he has prioritised the Lawrence smear claims.
In July he told MPs that his team found no evidence to back the smear allegations, although references were found to officers being deployed to look at supporters and campaigns around Stephen's murder.
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.''
Mr Francis has told The Guardian newspaper he will only give evidence to a public inquiry.