Members of UCATT will decide whether to merge with Unite during a vote which closes on November 1.

Unite is known for its more aggressive tactics during disputes and any merger could make life more uncomfortable for contractors and clients.

Both unions are already co-operating unofficially on the Crossrail projects where momentum is building over a series of site disputes.

The planned merger has been forced on UCATT following financial troubles at the construction union.

One observer said: “UCATT has been fairly toothless for a while now because of its money problems and leadership struggles.

“But forming a construction super union with Unite makes sense which will sound alarm bells with contractors who will be dealing with a very different beast in future.”

The UCATT National Delegate Conference in Scarborough backed the plan and Acting General Secretary, Brian Rye, said: “Now is the time to merge.”