The claim comes as thousands of construction workers are expected to stage a march in central London on Wednesday in protest over plans by seven major firms to withdraw from the JIB agreement.
Unite claim workers from all over the country will meet at the Shard site before marching on the Houses of Parliament.
The union is also preparing a strike ballot at Balfour Beatty Engineering Services which is seen as the ringleader of the breakaway seven who want to introduce a new pay and conditions agreement known as BESNA.
Protests against the move have become increasingly militant with sparks encouraging workers to leave sites to show their support.
The employers led by the umbrella body of the Heating and Ventilating Contractors’ Association believe unions are misrepresenting the terms of the new deal.
Association chief executive Blane Judd said: “The claims by the Unite union that the new proposed agreement will lead to what it calls massive pay cuts and workers heading for the sack are total fiction.
“No one will take a pay cut – in fact 30 per cent will see an increase in their pay packets – and no one will lose their jobs.
“The union is calling for a day of protest in London on 9 November and while I’m in favour of lawful marches I do hope that those considering coming to the capital have a strong grasp of the facts from the employers about the agreement and not just the union’s skewed views.”