Tesco was forced to close a number of its stores after a "routine IT upgrade" stopped tills from working at 100 shops ans supermarkets.
The technical glitch brought down checkouts just as Tesco launched a major loyalty scheme promotion in national newspapers to its millions of Clubcard holders.
Tills began to malfunction as they opened at 8 o'clock this morning, and stopped cashiers from being able to take customers' payments, a Tesco spokesperson said.
A number of the stores had to be closed. However others relied on self-service checkouts which still functioned but caused lengthy queues.
Tesco began to fix the problem around midday but some stores are still affected.
Stores were unable to take payments in central London, Birmingham, Liverpool, Kent, Shropshire, Hertfordshire, Cumbria, Worcestershire, Buckinghamshire, North Wales, and Northern Ireland, according to the Press Association.
Tesco, which has more than 2,000 stores in the UK, declined to comment on what technology was affected, but said it had to resort to asking store managers to reboot tills.
"A number of stores were affected by a routine IT upgrade this morning at various locations in the country," said a Tesco spokesperson.
"Our experts are fixing the problem as we speak ... we are very sorry for the delay and thank customers for their patience."
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