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An undercover policeman infiltrated a union to monitor protests against deaths on building sites, the Mirror can reveal.

Mark Jenner posed as a joiner and joined construction union UCATT.

Activists believe personal information collected by the officer was used by industry bosses to blacklist workers who raised concerns about health and safety.

Major industry chiefs benefited from the notorious blacklist that has been linked to Tory-donating construction firm Sir Robert McAlpine.

Steve Murphy, UCATT’s general secretary, said tonight: “Public money was spent on police covertly joining trade unions, infiltrating groups associated with trade unions and colluding with construction employers to blacklist workers.

"This is a scandal that must be exposed.”

Mr Murphy, who has demanded an inquiry, said the Met police “operated a secret organisation that destroyed innocent people’s lives”.

A Scotland Yard spokesperson said: “We neither confirm nor deny the identity of any individual alleged to have been in a covert role. We are not prepared to confirm or deny the deployment of individuals on specific operations.”

(Image: Daily Mirror)

While undercover in the 1990s, Mr Jenner – part of the Met’s now-disbanded Special Demonstration Squad – had a five-year relationship with an unsuspecting activist, Alison – not her real name.

She told the Mirror: “It is appalling they spied on people who were arguing for better health and safety at work.”

Mr Jenner also formed friendships with at least two building workers who ended up blacklisted. Labour MP Steve Rotheram said: “It’s very sinister. We were using an officer to infiltrate legitimate trade unions.”

Mr Jenner, 51, posed as Mark Cassidy, from Merseyside, and was a member of UCATT for three years starting in 1996.

He paid by direct debit and remained a UCATT member until 1998, during a period when the union was negotiating over a series of high profile construction projects including the Jubilee Line extension and the Millennium Dome.

Jenner’s diary, left when he fled the home he shared with “Alison”, show that he monitored meetings of UCATT and other unions. Alison says that union activity formed a “large part” of his day to day work.

He also penetrated the Colin Roach Centre in Hackney, East London, which campaigned on behalf of victims of police ill-treatment.

Three months ago UCATT used Freedom of Information laws to ask the Met: “Was there a policy of infiltration of trades unions conducted by the Special Demonstration Squad?”

Scotland Yard said it was not required to confirm or deny whether it held the details requested. The force give six reasons why they could not reveal the information including national security.

Blacklisting firm the Consulting Association was paid to supply information to construction companies. Sir Robert McAlpine stumped up £10,000 that was used to help launch the organisation in the early-90s. The association was closed down in 2009.

A probe by Derbyshire police found “no conclusive evidence” that Scotland Yard shared information with blacklisters.

But David Clancy, investigations manager at the ICO, has said the blacklisting files contained specific operational details “that I believe could only be supplied by the police or the security services”.

The Independent Police Complaints Commission said in 2013 that it was “likely that all special branches were involved in providing information”.

A second former SDS officer Peter Francis, who went undercover as activist Pete Black before turning whistleblower, has seen blacklisting files of two trade unionists he monitored and said they included details he had provided to his handlers.

The Mirror has spoken to two further active trade unionists who befriended Jenner and believe their files contain information he obtained undercover.

Jenner joined them at meetings of a small group called the Building Workers Group which was linked to UCATT and arranged pickets a construction sites where there had been deaths or serious injuries.

Steve Murphy added: “UCATT was infiltrated by the police and members have a right to know why. This sort of operation could only have been sanctioned at the highest level.

"I believe the truth rests with the Home Office. Who gave authority for the police to do this and how high did it go?”

Mr Murphy added: “The Metropolitan Police must not be allowed to hide the truth; they operated a secret organisation that destroyed innocent people’s lives. They must be held to account.

“It is increasingly clear that the only way we are going to get the full truth on the blacklisting scandal is by holding a full public inquiry which is open and transparent.”

Blacklisting firm the Consulting Association was paid to supply information to more than 40 construction companies, including Balfour Beatty, Skanska and Carillion, before it was raided by the Information Commissioner in 2009. Sir Robert McAlpine paid £10,000 to help set up the Consulting Association in the early 1990s.

Another covert police unit the National Extremism Tactical Co-Ordination Unit is known to have met with the Consulting Association in 2008, shortly before the blacklisting firm was closed down.

Handwritten notes of the meeting by Consulting Association boss Ian Kerr seen by the Mirror indicate that NECTU was set up to monitor animal rights activists but was “now expanding”.

The notes refer to the construction industry specifically and that NECTU wants to “liaise with industry”.

UCATT made a second FOI request to the Metropolitan Police about NECTU, asking for details of meetings with the Consulting Association.

Scotland Yard replied: “Searches failed to locate any information relevant to your request, therefore the information you have requested is not held by the MPs.”