Postal services were hit today when the Post Office's computer system went down.
Customers said they were unable to post parcels and were told that staff could not deal with anything which required a computer.
The system went down this morning, hitting branches across the country.
A Post Office spokesman said: "The Post Office apologises to its customers for the inconvenience caused by a computer problem currently affecting the service at branches across the UK.
"We are very sorry for any difficulties this has caused our customers. We are doing everything we can to fix the problem as quickly as possible and restore a full service to all branches.
"Branches are making special cash payments to Post Office Card Account customers. Post Office ATMs and Post & Go services in branch are unaffected. Bill payments and E-topups for mobile phones can be made at Paystation terminals in Post Office branches."
Andy Burrows, of Consumer Focus, said: "We are concerned that this is the fourth major service interruption in the Post Office's electronic systems in nine months. We will be meeting with Post Office Limited to understand how the problem will be addressed.
"The problems seem to be nationwide and have resulted in several hours of inconvenience for Post Office customers. Most branches have only been able to accept cash payments and do manual transactions such as selling stamps.
"We have heard some branches have decided to close early for the day - leaving customers without access to services. Customers need Post Office services, including the collection of benefits and pensions, to be reliable and resilient."
A Post Office spokesman said later: "Post Offices across the network are again working normally after a computer problem which had earlier affected branches nationwide for several hours. We apologise to all our customers for the service problems experienced in our branches and the inconvenience this may have caused.
"Services have now fully been restored and customers are able to complete all transactions across the Post Office network. We are continuing to monitor the situation closely to make sure our services remain available as normal.
"Post Office branches remained open and arrangements were put in place to ensure that special cash payments were made to pensioners and benefit claimants using the Post Office Card Account. Post Office ATMs, Post & Go services and Paystation bill payment and E-top up transactions were unaffected by this problem."