The union claim the companies are “acting in an anti-competitive way to drive down the pay and conditions of their workforces.”

The move is the latest round in a bitter battle between Unite and major M&E contractors over plans to quit the industry’s JIB pay and conditions agreement and implement the new BESNA deal.

One employer told the Enquirer: “Most of the workers have signed up for the new deal and this seems like the last desperate move by Unite.

“It’s fairly ridiculous and I’m sure the OFT will see the complaint for what it is.”

The HVCA said: “The HVCA has not been approached by the Office of Fair Trading and we’re not aware of any of the seven companies being approached either.

“We would call on Unite to sit down and talk to us about the introduction of the BESNA agreement rather than focusing on diversionary tactics.”

Unite claim the companies are breaking competition law by tearing up the JIB agreements and working together to set new pay and conditions under BESNA “which are not agreed by Unite.”

The move comes ahead of tomorrow’s strike ballot result across Balfour Beatty Engineering Services (BBES).

The seven construction firms including BBES are, NG Bailey Building Services, Crown House Technologies, Gratte Brothers, Spie Matthew Hall; Shepherd Engineering Services (SES) and T. Clarke Plc.

Unite national officer for construction, Bernard McAuley said: “These seven companies are trying to undermine long-standing agreements which have served this industry well for decades.

“These agreements acted as level playing field in the construction sector.

“By breaking away and paying significantly less, these employers will have a competitive advantage over other companies. We hope Balfour Beatty and the companies involved see sense in the face of the complaint we are making to the OFT.”