Thousands of customers were unable to send letters or parcels after the Post Office was hit by a “major outage”.
The postal firm, which has more than 11,500 branches, was hit by a huge IT blackout for hours as furious customers were turned away at the door.
The company said the outage affected its entire estate and was not cyber related, but did not specify what caused it.
One customer said: "I walked 25 minutes to the Post Office in flip flops for them to tell me the system is down and come back tomorrow."
Another added: "I walked to my local Post Office earlier today and they told me that they could not log into the system.”
A sign in Brighton read: “Brighton Post Office is CLOSED due to system crash. We are currently unable to process any transaction at this time. We hope to get this resolved as soon as possible.”
The majority of reported issues were in London, Birmingham and Manchester, according to outage measuring website Downdetector.
A Post Office spokesperson said: “There was a major outage across the Post Office estate this afternoon. We are pleased to announce that service has now been restored to branches.
“This is not a cyber-related issue. We are aware of what has caused this outage. We apologise for any inconvenience caused to customers and the operators of our branches.”
Earlier this year, Marks & Spencer lost around £300m after a cyberattack left shelves empty in some stores and online orders unfulfilled.
The British high street retailer took systems offline after being targeted by hackers around the Easter weekend - a move which had knock-on effect on its operations.
Customer personal data, which could have included names, email addresses, postal addresses and dates of birth, was taken by hackers in the attack.
Last week, three teenagers and a young woman were arrested in connection to the cyberattack. The arrests include three males – one aged 17 and the other two 19 – from the West Midlands and London.
A 20-year-old British woman from Staffordshire was also arrested last Tuesday.
