Scientists and engineers responsible for the closure programme at the sites in Oxford and Dorset say they were forced into industrial action after management refused to match the 2015 pay offer being awarded to the rest of the company.

RSRL and Magnox Limited merged on 1 April 2015 to form a single organisation operating as Magnox Limited, with Harwell and Winfrith now called Magnox South Sites.

But despite senior Magnox managers pushing the “one company ethos” at the former RSRL sites, staff at Harwell and Winfrith have been offered 1.2%, while those at the other Magnox sites receive 1.6%.

Prospect argues that the 1.2% increase will compound significant pay gaps that already exist between equivalent roles. In the worst cases, some Magnox South Sites staff will receive nearly £10,000 less than colleagues on comparable grades.

Despite a last-minute offer tabled by management, members of Prospect and Unite at Harwell staged a two-hour walkout on 1 December, which saw nearly 90% of staff take part. Colleagues at Winfrith followed suit the next day with more than 75% of staff involved.

Prospect negotiator Richard Tabbner said: “Members are furious and want a credible explanation as to why they cannot have parity with their wider Magnox colleagues. Even though the employer finally returned to the negotiating table, the new deal contains no new money.”

Further action is being considered, depending on the results of the re-opened negotiations.

Almost 75% of Magnox South Sites members voted in the ballot – 85% were in favour of a strike and 94% for action short of a strike.