Fraudsters using 'cloning' devices at a cash machine outside a large supermarket could have got away with thousands of pounds from scores of unsuspecting victims.

Police have received several reports of card fraud from people who have used the machine outside the Tesco store in the Oxford Retail Park in Cowley, but there are fears that many more people may have been hit.

The cloning device, a false front placed on the cash machines which then reads card details, may have been in place for as long as six days.

One victim, Klara Minier, of Fern Hill Road, Cowley, Oxford, said she was shocked and horrified to see £1,200 had been taken from her HSBC account after making a withdrawal from the machine outside Tesco.

The 25-year-old used her bank card at Tesco last Saturday. She then fell ill and says she did not use her card for three days but during that time more than 30 withdrawals were made from cash points in the London area - sometimes at a rate of two a minute.

She said: "I was shocked when I checked my bank balance online and saw so many withdrawals had been made which I knew I was not responsible for."

Ms Minier said the experience has left her afraid to use cash machines: "It's awful. I'm now broke. I'm just afraid to do anything at the moment and have found myself getting nervous about using my credit card in shops."

On the Monday before Ms Minier's card was cloned, another customer became suspicious of the machine and went to tell staff at the supermarket.

Mustapha, who lives in Barton but does not want to be fully identified, said: "I was walking straight to the security when these two guys came up.

"This bloke just came in my face, very angry, and said 'go.' He didn't want me to go back to the cash point or call security.

"He was trying to threaten me and putting his hand in his back pocket. It was like he was saying don't come any closer, I've got a knife.

He added: "The other one went to the machine and grabbed the device, and when the first guy saw the device had been taken he ran off."

Mustapha, who works at the nearby BMW factory, said many colleagues had had money taken from their accounts.

He added: "Since the incident I am really afraid to use the cash point. I prefer to use a machine inside a bank."

Thames Valley Police spokesman Tim Wiseman urged anyone who had used that machine in the last month to check their statements for unusual withdrawals.

He said: "I wouldn't want to speculate on how many people have been affected, but this is obviously a busy cash machine and, if these people have been returning to the site, there could well have been a high number of people using the machine that are potential victims."

Mr Wiseman warned people not to tamper with the devices, but to call police instead. He said: "These bits of kit are extremely expensive and these people will become dangerous if you try to take it off them."

HSBC, which operates the machine, confirmed that Ms Minier would be fully reimbursed.

A spokesman for Tesco said they had checked the machine as soon as they found out about the problem, but the device had already been removed.