Civil servants at 123 government departments and agencies will strike the day the chancellor delivers his Spring Budget, the PCS union has said.
Up to 100,000 officials will walk out on 15 March, in an escalation of the industrial action the union is staging over pay and conditions.
It will be the second mass strike since PCS began its action in December. Tens of thousands of civil servants walked out on 1 February, coinciding with action by other public service unions covering rail and teaching.
There have meanwhile been a number of targeted walkouts in areas including the Department for Work and Pensions, Border Force and National Highways. PCS has just announced further action by some of its members at the Driver and Vehicle Licensing Agency.
PCS is calling for a 10% pay rise, along with better pension conditions, job security and no cuts to redundancy terms.
The union's general secretary, Mark Serworka, told CSW last week that the union will not accept a deal that would give civil servants a higher pay rise than they are being offered now – 2-3% in most cases –in return for worsened conditions.
Announcing the March strike, Serworka said officials are "suffering a completely unacceptable decline in their pay".
"Rishi Sunak doesn't seem to understand that the more he ignores our members' demands for a pay rise to get them through the cost-of-living crisis, the more angry and more determined he makes them," he said.
Jeremy Hunt will deliver his second major fiscal event as chancellor on 15 March, following his Autumn Statement in November.
The statement made it impossible for departments to fund pay adjustments that matched – or came close to – the rate of inflation from existing budgets, without cutting jobs or services.
The Spring Budget could spell more spending pressures. Chancellor of the Duchy of Lancaster Oliver Dowden confirmed last month that the government is still committed to reducing the size of the civil service.
He said that while in former PM Boris Johnson’s cabinet, he had been "seized of the need" to bring the civil service headcount down to around where it stood after ministers started reducing it in 2010.
The "the only difference" in approach to Johnson’s plan to cut 91,000 jobs will be that “we will be driven by outcomes”, Dowden said – suggesting significant job cuts could still be on the way.
Serwotka said the PM could "end this dispute tomorrow if he puts more money on the table".
"If he refuses to do that, more action is inevitable," he added.
A government spokesperson said: “We greatly value the work of civil servants, and are committed to constructively engaging with unions, but the PCS Union’s demands would cost an unaffordable £2.4bn at a time when our focus must be on bringing down inflation to ease the pressure on households across the country.
“That is why public sector pay awards strike a careful balance between recognising the vital importance of public sector workers, while delivering value for taxpayers and being careful not to drive even higher prices in the future.”