BBC News business reporter
Management volunteers are being drafted in to keep big post offices open
The Christmas Eve walk-out by staff at big high-street Post Offices comes as the struggling service prepares for a major restructuring.
Union members have turned down their bosses' request that they should work an extra three-and-a-half hours on Friday 24 December.
It has been the custom and practice of post office workers since the 1970s to leave at 12:30pm on Christmas Eve, Andy Furey of the Communication Workers Union told the BBC.
The Post Office has been dogged by poor industrial relations.
It has had to restructure to combat dwindling revenues and losses of hundreds of millions of pounds between 2001 and 2003, and the changes - which involve 30,000 job cuts - have not always been popular with unions.
The union insists it recognises the need for changes as the Post Office faces up to market liberalisation.
Regulators are thought likely to bring forward deregulation, and the Post Office is considering closing up or selling off up to half of the 555 large post offices it runs directly.
Mr Furey said the latest action had no implications for the bigger picture. Pressure from the management to work till 4pm on Christmas Eve was "as an unnecessary and unjustified move," he said.
The two million customers who need to pick up benefits payments on Christmas Eve "work out at an average of 133 per [post] office and my members could serve them in 15-20 minutes," he said.
In his view, there would be no difficulty catering for all customers before a 12:30pm close.
"This is about the members having time off to spend with their families."
Unions and management work well on many topics, "But on this issue we could not come to a sensible conclusion", said Mr Furey.
Beating the competition
Post Office bosses say that the later closing time is a response to wider changes on the High St, with retailers staying open late on Christmas Eve and banks and building societies open.
"If you're on the High St and in retail, you have to be open when your customers want it," said David Mills, chief executive of the Post Office.
He said: "The union did not want its members to work for the hours they were paid."
The Post Office denied their was any element of grandstanding or antagonising the union over a matter of a few hours of working time.
Christmas Eve is one of the busier days of the year, with three day's worth of benefits to pay out because Post Offices will remain closed until Wednesday for public holidays.
Post Office Card Account holders include people collecting disability benefit and old age pensions.
"It is very important that these groups can get their money today," said Post Office spokesman David Simpson.
Mr Mills said: "We will have more than double the amount of transactions we would have in a normal day. This is about providing the service our customers need."
The National Pensioners Convention has backed a 12.30pm close, according to the CWU.
"I find it strange that a body that supports pensions is not in favour of it being easier to collect pensions," the Post Office spokesman said.
More than 98% of branches would remain open throughout Friday, despite calls by the CWU for strike action in 294 branches, the firm said.
Most of its 15,300 branches are sub-post offices and these will "maintain usual afternoon opening hours".
Nearly half the larger directly-managed post offices in High Streets will also be unaffected by the strike because, according to Mr Simpson, they have no union representation.
The remainder are due to be kept open by management staffing the counters.
The Post Office management is thought to be keen to hang on to its share of the benefits payment business in the face of competition from banks.
Branches like this may be open this afternoon, but some could be sold eventually
The government is thought to want to shift most benefit payments to banks but many people in vulnerable groups do not have bank accounts.
Mr Mills asked customers to check with their local branch, or telephone 0845 7 22 33 44 for information on finding their nearest available branch this afternoon.
The Post Office faces a major shake-up, though it has reversed huge losses and reported profits of £220m in the year to May 2004. It lost £200m in the previous financial year.
However, the so-called Crown post offices which are at the centre of the walk-out remain loss-making, to the tune of £70m in year to May 2004.
During 2005, industry regulator Postcomm is expected to announce a deadline for deregulation.