The union said its members in 124 government departments and other bodies will walk out on February 1.
Shares rose in London after a rollercoaster day.
The influential S&P Global/ CIPS flash UK composite purchasing managers index showed a reading of 47.2 in October, below September’s 49.1.
Lord King of Lothbury outlined the challenge for the next prime minister to confront high inflation.
The Labour leader also urged disillusioned Conservative MPs to put the interests of the British people first.