Police gathered intelligence on trade union activists and passed the information to a clandestine blacklisting agency that unlawfully stored secret files on thousands of workers, according to evidence seen by the Guardian.

Peter Francis, the whistleblower who revealed that police spied on supporters of the Stephen Lawrence family, says he believes that he personally collected some of the intelligence that later appeared in the files of the blacklisting agency.

The former officer said that while he was embedded in the campaign Youth Against Racism in Europe, he gathered – at the request of his police supervisors – information about the marriage of an American political activist whom the Home Office wanted to deport in the 1990s when the authorities were arguing she was in a sham marriage.

A file was kept by the blacklisting agency on a bricklayer called Frank Smith, who was in a relationship with the American, contained information about her and recording the allegation that she was in a fake marriage. Other campaigners say that allegation was an excuse to remove a left wing activist from Britain.

The agency's records on Smith also note that they had been told he was under surveillance by the state – with the file noting that Smith was "under constant watch (officially) and seen as politically dangerous".

The agency – funded by Britain's biggest building firms – prevented workers they deemed to be politically troublesome from getting jobs in the construction industry. Many trade unionists say their lives were ruined for years when they were barred from getting work as a result.

The blacklisting agency – known as the Consulting Association – compiled files on workers in the construction industry that logged details of their workplace activities such as the names of companies they worked for, their support for trade unions, and their disputes with managers.

However, previously confidential files – seen by the Guardian – reveal how the blacklisting agency also secretly recorded the political activities of the trade unionists when they took part in demonstrations utside their workplaces against causes such as fascism.

The testimony from the former undercover officer about the information he gathered on the personal lives and protest activities of the trade unionists is disclosed as a team of police officers led by Derbyshire Chief Constable, Mick Creedon, investigates the conduct of the Special Branch unit that planted spies in political campaigns for four decades.

As part of its inquiry, Creedon's team is examining claims that police colluded with the blacklisters and shared intelligence from its own files on so-called "subversives".

The new evidence has emerged after senior figures involved for many years in running the blacklisting admitted to a parliamentary committee that the blacklisters regularly exchanged information, or had contacts, with the police.

Francis has been disclosing details of his covert work for the unit, the Special Demonstration Squad, as he supports calls for a public inquiry into its infiltration of political groups between 1968 and 2008.

He has helped reveal how the SDS spies stole identities of dead children, monitored groups campaigning for a proper investigation into the murder of Stephen Lawrence, and routinely formed relationships with women they were spying on.

The new files are drawn from a database compiled by the Consulting Association.

For 15 years, the agency was funded by more than 40 major firms in the construction industry whose managers fed information about individual workers to the Consulting Association.

Pooling this information, staff at the association compiled files on more than 3,200 workers in an anonymous office in Droitwich, Worcestershire. When workers applied for jobs, managers contacted the association to check what information was held on them and would then decide whether to give work to them.

The files, some of which were inherited from a predecessor blacklisting agency called the Economic League, contained comments such as "militant ringleader", "agitator", "is a good worker but has proved to be very militant", "do not touch", and "that subject is a very bad troublemaker and would not be re-employed".

Creedon's team is now investigating claims from the blacklisted workers that portions of the files could only have come from the police or security services, as managers in the construction industry could simply not have known about demonstrations the trade unionists were attending outside of their work hours.

Files maintained by the blacklisters on three trade unionists logged how they were "observed" or "apprehended" while protesting against fascists laying a wreath at the Cenotaph in commemoration of Britain's war dead on Remembrance Sunday in 1999.

The files on two trade unionists record a piece of information that they say was known to only a handful of members at the heart of the Youth Against Racism in Europe (YRE) group they were active in during the 1990s.

Francis said he collected this piece of information and reported it back to his Special Branch bosses. The information was that they were part of a loose grouping of anti-racist activists, known colloquially as the "away team", who protected YRE demonstrations from physical attacks by far-right activists.

The Scottish Affairs select committee that is investigating the blacklisting has uncovered documents showing that the police unit monitoring political activists met the blacklisting agency in 2008 to discuss sharing information.

The blacklisting agency was closed in 2009 after the data watchdog, the information commissioner, declared it was amassing files unlawfully.

Dave Clancy, the senior investigator from the watchdog that raided and closed down the blacklisting agency, also testified that some of the details in the files he seized would have been supplied by the police or the security services.

An ex-police officer, Clancy said the wording of information about vehicle registrations and deportations appeared to originate from police records.

At least 80 blacklisted workers are preparing a high court action against construction firms for lost earnings after gaining access to their files.

Dave Smith, spokesman for the blacklisted workers, said: "Policing investigating the police is never going to get to the truth. This smacks of a cover-up. Only a judge-led public inquiry will get to the facts of collusion between the state and the blacklisters."

A Metropolitan Police spokesman said: "A complaint alleging breach of the Data Protection Act by police was referred to the Independent Police Complaints Commission who directed it should be subject to a local investigation. The investigation is at an early stage and we are not prepared to provide further details at this time."