About 100,000 civil servants are to strike on the day UK Chancellor Jeremy Hunt unveils his spring Budget.
The Public and Commercial (PCS) union said members would walk out on March 15 as part of an ongoing row with the government over pay and conditions.
It has been calling for a 10% pay rise, better pensions, job security and no cuts to redundancy terms.
The government said the union's demands would cost an "unaffordable £2.4billion".
The PCS represents thousands of workers who work in government departments as well as workers at organisations such as Ofsted, the Maritime and Coastguard Agency, and Border Force.
The union said its members from 123 government departments would take part in the industrial action, which include staff at the Department for Education, Department of Health and Social Care and the Home Office.
Previous action
Some 100,000 civil servants previously walked out on February 1.
Mark Serwotka, general secretary of the PCS, said its members were "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 the prime minister could "end this dispute tomorrow if he puts more money on the table".
"If he refuses to do that, more action is inevitable," Mr Serwotka added.
The government has previously offered civil servants a 2% to 3% pay rise, but union bosses have been calling for 10% - a similar level to the rate of overall inflation in the UK, which is at a 40-year high.
Wave of strikes
Price rises have been squeezing household budgets and there has been a wave of strikes across Britain in recent months.
The government told the BBC it valued the work of civil servants and added it was "committed to constructively engaging with unions".
"But the PCS union's demands would cost an unaffordable £2.4billion at a time when our focus must be on bringing down inflation to ease the pressure on households across the country," a spokesperson said.
"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."