Transactions were affected at the Post Office network on Monday as computer issues caused problems on one of the busiest days of the year.
Some transactions were affected for two 30-minute periods, leading to queues.
The Post Office apologised, saying that the service had now been restored across the UK network of 11,800 branches.
The problem came as Royal Mail's website continued to suffer from a computer glitch.
"We are very sorry for the inconvenience this problem has caused customers. We know how important the Post Office is to our customers at all times, but especially at Christmas," said Kevin Gilliland, the Post Office's network and sales director.
This is the second computer problem at the network in six months. In July, a shutdown blocked access for 3.3 million customers with Post Office card accounts.
Some pensioners have their benefits paid into a Post Office card account.
A Post Office spokesman said the glitch affected most services offered by the branches which were hit.
Andy Burrows, from the government-backed watchdog Consumer Focus, said the problems would have caused "major disruption".
"People rely upon the Post Office not just for mail services but also to access cash and much-needed pensions and benefits. We will be taking up our concerns with Post Office Limited as a priority," he said.
Post offices were expected to be particularly busy, because Royal Mail was predicting its busiest day of the year.
They were expecting 131 million items to be posted, double the usual number. They had taken on thousands of extra staff to cope with the Christmas rush, caused in part by the popularity of online shopping.
Last week, Royal Mail admitted that it had charged some customers twice for postal services as problems continue to hit its website.
Some 600 customers had cards debited twice, at an average of £50 each time, after repeating a purchase in the SmartStamp section which the system told them had initially failed. Various online postage applications - such as Price Finder - have also been down since 21 November.
A Post Office spokesman said the issues affecting Royal Mail were unrelated to the problems at the Post Office network on Monday.
The final deadline of Christmas post for sending overseas was Monday, and for standard packages is 14 December.
The Christmas deadline for second-class post is 17 December and is three days later for first-class deliveries.