Tesco self-service tills suffer from an apparent "security loophole" making it easy for fraudsters to steal cash, consumer group Which? has said.
Tesco says fraud levels are low
Tesco operates the check-outs at 320 stores, allowing shoppers to scan goods and pay with a debit or credit card.
But the tills do not take chip-and-pin card technology and Which? says this allows thieves to use stolen cards.
In response, Tesco said fraud levels at the tills were low and chip-and-pin technology would be in place soon.
Which? said it was prompted to issue the warning after several consumers contacted it after having fallen victim to fraud.
The editor of Which?, Neil Fowler, accused Tesco of being "highly irresponsible" for not having adapted the tills to chip-and-pin technology.
"It is hard to believe that the UK's biggest supermarket is making it so easy for criminals to use stolen credit cards in their stores," Mr Fowler said.
"We can only hope they close this obvious loophole and introduce standard security procedures in the very near future."
Tesco denied that it had a significant fraud problem at its self-service tills.
"While it is impossible to stamp out all credit card fraud, fraud levels at our self-service checkouts and petrol pumps are very low and no higher than at our main checkouts, which have full chip-and-pin," a Tesco spokesman told BBC News.
The supermarket said it had already started to introduce chip-and-pin at its self-service tills.
"When we originally introduced the self-service tills over two years ago, chip-and-pin was not a proven technology.
"Now that it has become widely accepted we will be rolling it out into all our 320 current installations - this process has already begun and should be completed by December."