Cleaners, security guards and support staff in three Whitehall departments will launch a five-day strike on Monday in a dispute over pay.
Members of the Public and Commercial Services union (PCS) employed by ISS will walk out at the Department for Energy Security & Net Zero, Department for Business & Trade and Department for Science, Innovation & Technology.
The union said the workers are angry at being offered a below-inflation 2.2% pay rise.
“Our members are working alongside people earning more money, on better terms and conditions, while the company is making a vast profit at their expense.
“It’s time for ISS to pay our members a fair wage and time for the Government to end outsourcing and the resulting two-tier workforce.”
The new action follows a strike last month.