Construction union Unite has been in dispute with Mears and joint venture company Manchester Working since May staging a series of short-term strikes.

Now workers are threatening to walk out for four weeks from Saturday 8 July to Friday 4 August.

The move has mystified Mears who said they have offered 10% pay rises in a bid to settle the dispute.

A Mears spokesperson said: “We are disappointed in Unite’s unnecessary action.

“Unite has shown no interest in negotiation and has not put forward any proposals following their rejection of our offer.

“Despite their action, we have maintained services with little or no disruption to customers or clients, who have been very understanding.”

Unite regional co-ordinating officer Andy Fisher said: “Unite is stepping up its strike action in response to Mears’ failure to enter into negotiations to resolve the dispute and is a direct consequence of the company’s attempts to undermine our industrial action.”

Unite said that longstanding problems regarding pay differentials result in workers being paid up to £3,500 less than colleagues for undertaking the same work.

Fisher added: “Our members have only taken industrial action as a last resort. Mears has had years to resolve this issue and end the inequality and injustice faced by our members but has failed to do so.”