Hundreds of managers at Crown Post Offices will go on strike today in a row over pay.
Unite, which represents 900 managers, warned that the action will hit services at the offices, the larger branches, usually sited on high streets.
The union said its members have not received a pay rise since June 2011 and accused the Post Office of "dragging its feet."
Unite official Brian Scott said: "While we understand that the industrial action will cause inconvenience to the public, the ball is in the management's court - as it has been for the last 16 months - to reach a fair and equitable settlement.
"Unite is ready to have talks at anytime with the Post Office to resolve this matter, but the management has taken a very hardline stance against our members who have seen their living standards seriously eroded."
Counter staff belonging to the Communication Workers Union have taken a series of strikes this year in a separate row over pay, jobs and branch closures.
They were due to strike again today, but it has been called off to allow talks to be held.