A major IT problem at the Post Office has hit branches nationwide, stopping many standard postal services.

The Post Office has not yet explained the nature of the problem, which began this morning and led to some branches having to close. It is not clear if the root of the problem is with the Horizon system used by its branches. That system experienced serious problems, flooring transactions for two 30 minute periods, last December.

Customers told the BBC that they were unable to post parcels or do anything else requiring staff to use a computer.

The Post Office said in a statement that it "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."

It added: "Post Office ATMs and Post & Go services in branch are unaffected. Bill payments and e-top-ups for mobile phones can be made at Paystation terminals in Post Office branches."

In July, another computer problem blocked access for 3.3 million customers with Post Office card accounts. It is not yet known if the issues are connected.