Building of Hinkley Point C nuclear power station near Burnham-On-Sea could be delayed again, after trade unions for construction staff working on the £20bn project this week announced a ballot for strike action in a dispute over pay.

More than 95% of members balloted by the GMB and Unite unions rejected a pay increase offered by the French energy firm EDF and its contractor Bylor following months of discussions.

Any extension of the labour dispute risks further time and cost overruns for what is Europe’s largest construction project, which is already running over budget and behind schedule.

Separately, staff at outsourcing group Capita are to go on strike for six days from 5th October amid changes to the company’s pension scheme that will result in a ‘massive cut’ to their retirement income, says Unite.

Earlier in the year, EDF announced that the cost of Hinkley C had risen from £18bn to £20.3bn and that construction of the plant would also be delayed by 15 months, adding that it might not start producing electricity until 2027.

Strike action was initially called off in June following an interim deal over bonus payments being reached in the hope that a permanent agreement avoiding any further delays at the site could be reached.

Talks have broken down and both GMB and Unite now intend to ballot members over strike action regarding the employment terms of more than 1,000 civil engineers.

The main complaint is that the civil engineering contracts are “significantly below” the rates of workers on mechanical and engineering deals.

“Members have made their views clear; the unions warned the amount of money being offered was not sufficient and this has proved to be the case,” said Jerry Swain, Unite’s national construction officer. “The unions are fully prepared to return to the negotiating table if an improved offer is put forward.”

EDF Energy says it’s “disappointed” and that it had offered increased pay, bonuses and benefits as part of a package that was “fair and reasonable”. “The terms and conditions of employment at Hinkley Point C are superior to anything you will find in the UK construction industry,” it adds.