Hundreds of main post offices will be hit by the Union of Communication Workers' stoppage in protest at "backdoor privatisation" of the service.
Managers said most of the country's 20,000 post offices will be open, despite the 24-hour walkout by counters staff.
Richard Dykes, Post Office Counters managing director, attacked the union for calling the strike on a main benefit collection day for child allowances, pensions and income support payments.
"The union's threat to close these post offices is a potential attack on the most vulnerable members of the community just before Christmas - it's absolutely disgraceful," he said.
Despite the industrial action, the Post Office is heading for a record Christmas.
About 90 million letters and cards are believed to have been handled on Friday last week, compared with a daily average of 64 million.
Yesterday the number of letters and cards collected is estimated to have more than doubled to 13 million from an average 6 million on a normal Sunday.
About 115 million items are expected to go through the system tomorrow, which is anticipated to be the most hectic day of 1994.Reuse content