IPCC tells lawyers representing victims it is likely that all special branches were involved in providing information

Police officers across the country supplied information on workers to a blacklist operation run by Britain's biggest construction companies, the police watchdog has told lawyers representing victims.

The Independent Police Complaints Commission has informed those affected that a Scotland Yard inquiry into police collusion has identified that it is "likely that all special branches were involved in providing information" that kept certain individuals out of work.

The IPCC's disclosure confirms suspicions voiced by the information commissioner's office last year that the police had been involved in providing some of the information held on the files, as revealed by this newspaper.

The admission has been welcomed by campaigners for the 3,200 workers whose names were on the blacklist that was run for construction companies as "absolute evidence" of a conspiracy between the state and industry that lasted for decades.

Dave Smith, an engineer who had a 36-page file under his name and was repeatedly victimised for highlighting safety hazards on sites, including the presence of asbestos, said he was delighted that the IPCC had revealed "the truth". He added: "For the past five years, when we have been saying the police were involved, we were told we were talking nonsense and it was a conspiracy theory. They wanted it to go away. Now we have the absolute evidence and this is no longer about industrial relations but is a major human rights scandal involving a conspiracy between the police and the industry."

The blacklist, run by a company called the Consulting Association, funded by 40 major firms in the construction industry including Balfour Beatty and Sir Robert McAlpine, was discovered in 2009 after a raid by the information commissioner's office. Since then, the victims have fought to find out who was providing information against them. The IPCC's correspondence is regarded as a major breakthrough.

However, the watchdog's disclosure has been disputed by a subsequent letter to the victims' solicitors. This was sent by a recently appointed senior investigating officer for the inquiry into the activities of undercover police officers, known as Operation Herne.

In a letter, seen by the Observer, detective inspector Steve Craddock insists that the IPCC's statement is incorrect and that he has seen "no conclusive evidence" that Scotland Yard shared information with the blacklisters.

The IPCC is standing by its correspondence, which it says was informed by discussions with the Metropolitan Police and that "developments since that ... are a matter for the Metropolitan Police".

In response, a spokesman for Craddock said Operation Herne's investigating officer was "aware of the apparent contradiction and is looking into how that may have arisen". She added: "Operation Herne will report on the 'blacklisting' matter to the Metropolitan Police commissioner in due course."

The developments come as the group fighting for justice for the blacklisted workers has received confirmation of a meeting between undercover police officers and those running the blacklist in November 2008. The information commissioner's officers have confirmed in a freedom of information response that they hold notes from a meeting between the Consulting Association and officers from the police national extremism tactical co-ordination unit, which runs undercover officers.

The notes of the 2008 meeting are part of a haul of documents seized by the information commissioner's office when it discovered the existence of the secret blacklist during a raid on an office in Droitwich, Worcestershire.

Sir Robert McAlpine, which was allegedly a major player in the establishment and funding of the blacklist, is currently being sued in the high court over an unlawful conspiracy to amass a database of information against thousands of people.

Last week, in a dramatic twist, eight major construction companies, including Sir Robert McAlpine, announced that they would compensate some of the 3,213 workers whose names had been on a blacklist.

A statement said: "The companies – Balfour Beatty, Carillion, Costain, Kier, Laing O'Rourke, Sir Robert McAlpine, Skanska UK and Vinci – all apologise for their involvement with the Consulting Association and the impact that its database may have had on any individual construction worker."

Sean Curran, a solicitor representing 69 victims in the high court, said he cautiously welcomed the announcement but raised concerns over the involvement of the unions, which are also suspected of providing information to the blacklist operation in some cases. He said: "We note that there has been reference to the consultation of Ucatt and Unite in the formulation of the proposed compensation scheme. We express serious concern about the involvement of those organisations.

"We have seen evidence that implicates Amicus (which evolved into Unite) and Ucatt officials in the supply of negative commentary about the suitability of their members for employment. That commentary frequently made its way onto the Consulting Association database and was no doubt one of the factors that led to denials of employment.

"It is also worthy of note that those unions refused to support their members in bringing a High Court claim so that they could seek redress for the hardship that they suffered. Many of those that we represent are firm that they object to Unite or Ucatt playing any part in negotiations with the relevant companies for these reasons."

Claire Windsor, solicitor for the victims in regard to the complaint over police collusion, said her clients had lost any faith in the ability of the police to investigate themselves and that the blacklist support group was now calling for a judge-led independent inquiry into blacklisting.