Doreen Lawrence: "Now I just don't know what
to believe any more."
The mother of murdered Stephen Lawrence says she has "no confidence" in the ability of police to investigate claims officers tried to carried out a smear campaign against her family.
Addressing the Home Affairs Select Committee, Doreen Lawrence said the claim, by an ex-Met officer 20 years after Stephen died, had shocked her.
She said it was "not transparent" for police to investigate other police.
Two existing inquiries are to examine ex-officer Peter Francis's claim.
Mrs Lawrence called for a public inquiry after former Mr Francis said he posed as an anti-racism campaigner following her son's murder in 1993 and was asked to find "dirt" on the family.
She told MPs that the "drip-drip" of information about police action surrounding Stephen's death had to be replaced by a judge-led public inquiry with witnesses being called.
"Over the years, I was beginning to have a level of trust, we had the investigation and the court case.... I was beginning to have some confidence.
"Now I just don't know what to believe any more."
There is already a police investigation into the activities of undercover officers, called Operation Hearne.
Another inquiry, into corruption in the original investigation into Stephen's murder, is being led by Mark Ellison QC.
Speaking about the latest allegations, she said: "We had no idea whatsoever. In hindsight, now I know why [police liaison officers] were questioning our family and friends for their contact details."
She said the family had been uncomfortable with the liaison officers as they could not understand why they were not updating them on the inquiry as they had expected.
She said she once asked police why the officers were seeking details, and was told that in murder cases it was frequently family or friends who were involved.
"I pointed out that our family and friends were black, and Stephen's killers were white."
It took more than 18 years to bring two of Stephen's killers to justice. An inquiry following the murder led the Metropolitan Police to be accused of institutional racism and found failings in how the force had investigated the crime.
Mrs Lawrence said: "There are still elements of racism within the police... and I don't think all the lessons have been learned."