Cleaners and security guards who work at the Department for Energy Security and Net Zero are to strike over pay later this month, the civil service's biggest union has announced.
PCS said the strike decision followed a ballot of members employed by outsourcing company ISS at the department.
The union said the strike would last for five days, starting from 22 July, and cover the last two days before parliament goes into recess – meaning ministers will still be using the building.
As well as a pay increase to £17 an hour, PCS said the approximately 60 staff are seeking improvements in terms and conditions that would "move towards alignment" with those of civil servants employed by DESNZ.
The strike is intended to put pressure on the new Labour government to bring the ISS staff back in house when the current ISS contract with the Government Property Agency ends next April.
PCS general secretary Fran Heathcote said the way the demands of the ISS staff were handled would be a test of the new Labour government's "good jobs" agenda.
"This is our first ballot result since the election of the new government and gives Labour an early chance to deliver one of their manifesto pledges – the insourcing of public contracts," she said.
"The shameful disparity in pay between two groups of workers employed in the same building shines a light onto the murky world of outsourcing.
"Our members are standing by to suspend their strike if the government keeps its word and brings this – and other – public contracts back in-house so all workers are treated the same and receive fair pay."
An ISS spokesperson said the company was "disappointed" that the strike was going ahead.
"We value the contribution of every ISS team member and will continue to work towards a resolution," they said. "We have robust plans in place to ensure continuity of service."
CSW understands that GPA is keen for the contractor and the union to cooperate to resolve the dispute.