Controversially, those who received a “Must Improve” performance rating will not receive a pay increase – affecting around 10% of staff.

Prospect negotiator Julie Flanagan said: “The performance system in the Cabinet Office has shown discriminatory outcomes year on year.

“The process is widely discredited because statistics consistently show that the system discriminates against staff with protected characteristics, including disability, working pattern and age.

“Despite our protests, and requests to reconsider the decision to withhold pay, the Cabinet Office Executive Committee decided to go ahead with this offer. It beggars belief that a government employer would make a formal pay offer knowing that it is discriminatory”.

Staff who were given an “Exceeded” performance rating will also receive performance bonuses of up to £2,600, dependent on grade.

Prospect has rejected the offer and has urged members to make contact so that the union can help them challenge any discrimination.