Major steelwork contractors in London this week joined forces over pay, amid fears of a union push for increases at sites across the capital.

The firms held a meeting convened by the Engineering Construction Industry Association to voice their concerns over demands, which, if accepted, would exceed those in the usual “blue book” normally applicable on construction projects. The initiative by the employers comes as steelworkers at Heathrow Terminal 5 were balloting over strike action on bonuses that would breach the agreement.

There are fears among steel contractors that the unions want a London-wide £1-an-hour bonus for steelworkers next year. This would mean a return to measures won under a strike by Canary Wharf steel erectors in the 1980s but scrapped during the recession in the early 1990s.

A spokesperson for the association said: “Considerable concern was expressed about the apparent lack of support of the Amicus and GMB for the blue book agreement. The association’s managing director wrote to both unions earlier this month urging them to clarify their position, but so far has received no reply.”

A senior source at Amicus said that the union would push for increased pay across London. He said: “London steel erectors have so far never been able to get their £1 an hour back.

“You get it on big jobs such as Swiss Re but not on others. There are a lot of high-rise projects scheduled for London, and we will go after the deal.”

As Building went to press on Wednesday, union officials were scheduled to meet T5 steel contractors Watsons and Fast Track in a bid to secure a £1-an-hour bonus, as worked balloted over strike action.

Watsons said in a letter to its workers that the company was “unlikely to accept” demands, which are not payable under the blue book agreement. In the letter, Watsons also said that a vote for industrial action “could seriously affect the company’s ability to secure future work in the UK”.