Thousands of workers will launch a wave of strikes this week, hitting postal services, rail companies and airlines in the run-up to Christmas.
About 3,000 staff at hundreds of Crown post offices will walk out on Monday, Tuesday and Friday.
The action marks a major escalation of their dispute over pension changes, job security and closures.
They will be joined on Wednesday and Thursday by workers who supply many sub-post offices with cash.
Kevin Gilliland, Post Office group network and sales director, said fewer than 300 branches would be affected, with “business as usual in almost all of our network”.
There are fears the strikes could escalate if unofficial action is taken by Royal Mail workers who refuse to cross picket lines.
A Royal Mail spokesman said: “There will be little or no impact on Royal Mail as a result of the CWU strike at the Post Office. Deliveries will carry on as normal and the last posting dates for Christmas remain unchanged.”
Meanwhile, RMT members at Southern Railway will strike on Monday and Tuesday in a dispute over the role of conductors.
Along with a continuing ban on overtime by drivers in Aslef, the action will cause more misery for Southern’s 300,000 passengers.
RMT general secretary Mick Cash said: “Our conductor members on Southern are on strike this week in defence of the safety of the travelling public and that remains the focus of the entire union.”
Southern was accused of cancelling trains and blaming industrial action even though drivers were available, for the second consecutive weekend. The company has denied Aslef’s claim.
British Airways cabin crew belonging to the Unite union are due to strike on Christmas Day and Boxing Day in a row over pay.