Post Office and rail workers have gone on strike in a week that could also see industrial action disrupting the airline industry.
A strike at Crown post offices has closed about 50 High Street branches.
Meanwhile the rail strike at Southern continues with conductors beginning two days of action.
Talks aimed at averting a strike by British Airways cabin crew over Christmas have started at the conciliation service Acas.
There are 300 Crown post offices across the UK, larger branches mostly situated in city centres. The Post Office will be updating its website with details of branches affected by the strike.
It says 250 branches have opened, but the union questions those figures and says even if a branch is open, there may be a reduced service.
Workers at Crown post offices are protesting against pension changes, job security and closures.
On Wednesday and Thursday, delivery drivers who supply many rural offices with cash will join the action.
There are fears the situation could escalate if unofficial action is taken by Royal Mail workers – who are not currently involved in the dispute – and they 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.”
Post Office communications officer Mark Davies said that the company was trying to become more efficient and reduce its losses, which would be good news for the taxpayer.
“We have reduced losses from £120m four years ago, to £26m in the last year, and we hope to break even next year. We don’t believe that taxpayers should pay out to support the Post Office, ” he said.
Analysis: John Moylan, Industry correspondent
I can’t remember a week like it.
The Acas conciliatation service, which helps arbitrate disputes, is meeting BA workers on Monday, baggage handlers on Tuesday and they are trying to work with the Post Office behind the scenes too. There’s potential for a lot of disruption across the country.
In the Post Office dispute there’s concern that Royal Mail staff may not be prepared to cross picket lines when they go to pick up parcels.
If that happens, then their managers will have to take action and that could lead to unofficial lightening strikes. I’m told that Liverpool and Glasgow are likely to be the flash points if that does happen – but it all depends how this week plays out.
Meanwhile, guards at Southern rail 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.”
British Airways cabin crew belonging to the Unite union are due to strike on Christmas Day and Boxing Day in a row over pay.
Talks aimed at averting the strike have got under way at the conciliation service Acas.
In addition, Unite members employed by Swissport as baggage handlers and other ground staff at 18 airports across the UK are set to walk out on Friday and Saturday over pay, although talks will also be held at Acas on Tuesday.
Members of the Transport Salaried Staffs Association and RMT on London Underground are continuing with an overtime ban in a dispute over jobs and ticket office closures, while pilots at Virgin Atlantic will start a “work to contract” action from Friday in a row over union recognition.