Every little helps in these belt-tightening times - but customers at Tesco in Covent Garden received cold comfort from the supermarket giant after it over-charged for special offer groceries.

The nation's largest supermarket, which earned an estimated £5.5million each day last year, blamed this week's mistake on an in-store IT glitch which meant promotional prices did not show up on customers' bills.

Hundreds of angry shoppers at the flagship Tesco Metro store, in Bedford Street, near The Strand, noticed the error and queued for refunds. But the company allowed the mistake to go unchecked for a week were slow to install signs warning of the glitch.

Over-payments are likely to have gone unnoticed by many regulars at the store.

"It's shocking that a multi-national would not tell its customers that it is over-charging them," said one disgruntled regular, who did not want to be named. "You buy special offers on trust that you're getting a bargain - this week we've all been paying over the odds. It's unbelievable - if I had a choice I would go to ASDA but there isn't one nearby."

The customer said she bought a melon on promotion from £1.48 to 65p, but was charged the full price at the check-out.

"When I told the staff, they said it had been happening for the whole week. I was shocked at how casual they were about it, because in Covent Garden there will be thousands of people passing through the doors every day who don't check their receipts."

A company spokesman apologised a mistake had been made, said notices warning customers to check their bills will be put up in store following the enquiry from the London Informer and urged affected customers to keep their reciepts and claim a refund.