Special Branch officers gathered intelligence on anti-racism campaigners from within the official inquiry into the murder of Stephen Lawrence, according to a police whistleblower.

Wearing plain clothes, the Special Branch officers regularly attended the hearings of the Macpherson inquiry to collect intelligence on witnesses and activists who supported the Lawrence family's campaign for justice, it is claimed.

Peter Francis, a former police spy during the 1990s, said that the officers' role was to "gather intelligence on individuals and groups attending the public inquiry".

"The reason was to further identify those deemed to be subversive. It was an absolute golden intelligence opportunity to work out who the deemed subversives were in the anti-racist groups."

His claim adds to allegations that at least two other police forces were involved in spying on those giving evidence to the landmark inquiry headed by Sir William Macpherson, a former high court judge. Last week Theresa May, the home secretary, instructed all police forces in England and Wales to conduct an "urgent but exhaustive" search of their records to establish whether they had gathered intelligence on Lawrence's family or the Macpherson inquiry. The deadline expired on Wednesday.

Doreen Lawrence, the mother of the murdered teenager, made it plain on Wednesday that there should be an independent inquiry into claims made by Francis that his superiors asked him to find "dirt" that could be used to smear her family after her son was stabbed to death in a racist attack in 1993.

Francis was a member of a covert unit within Special Branch, the Special Demonstration Squad, between 1993 and 1997 when he infiltrated anti-racist groups. He has previously said that he had argued that details of his and other undercover deployments should have been disclosed to the inquiry when it was set up in 1997 to examine the police failure to catch Lawrence's killers – but he was overruled by his superiors.

One reason the Special Branch officers were sent to the inquiry was to gather any advance warning that the undercover operation was about to be exposed, he said. "When the Macpherson inquiry was going on, they had meetings with me to reassure me that 'everything's totally fine, we are going to find out if there's even a sniff of you out there'."

Francis made it clear that the officers were not long-term undercover spies, but posed as interested members of the inquiry in the public gallery. "I am 100% aware that the Metropolitan Police Special Branch had a Special Branch officer regularly, if not daily, in both parts of the Macpherson inquiry."

The inquiry took evidence from an array of grassroots anti-racist activists and organisations, such as local race equality bodies, on their views on racism within the police.

"[Special Branch] regarded it as an intelligence coup, to have all these people to go along there, which means that they would have almost an A to Z list of activists who were involved in the race campaigns locally," Francis said.

Last week the Independent Police Complaints Commission was asked to examine claims that Sir Norman Bettison, then a senior officer in West Yorkshire police who went on to become its chief constable, orchestrated an alleged attempt to undermine a community worker who was about to denounce police racism in his area at one of the hearings. The inquiry also held hearings in Manchester, Bradford, Birmingham and Bristol.

The IPCC has also been asked to look at claims, uncovered in an internal memo, that the Greater Manchester Special Branch wanted intelligence on "groups and individuals" likely to attend another hearing.

On Thursday, the Met said it was investigating the claims that Special Branch gathered intelligence at the inquiry. A spokesman said: "There have recently been a number of questions put to the Metropolitan police with regard to police activity around the Stephen Lawrence investigation and Macpherson inquiry. While these issues relate to events that took place in the late 1990s and early 2000s, we are taking them seriously and are undertaking research to understand their accuracy, extent and context."