Man goes into Kiran Patel's store, (Coulsdon News in Surrey) and asks for a £100 Ukash voucher, which Kiran put through his Payzone terminal. The customer didn't touch the voucher (neither did he have a mobile with which to photograph it), but then decided he had left his wallet in the car and went to get it. He didn't return and left the voucher behind. This was a Sunday. On Monday Kiran rang the helpdesk and gave them the reference number to cancel the credit.

Credit usually comes within one month, but in this case it didn't and when he queried it he was told that the "voucher had been used".

He says: "Only two people can know the voucher Pin number the retailer and Ukash."

He added that one of his regular suppliers (of sundry goods) told him that he had heard of this happening in quite a few shops. The suggestion was 50. He put Kiran in touch with Prem Kumar, who runs Food & Wine, also in Coulsdon. He has a PayPoint terminal and similarly lost £100 on the same day.

I rang him to confirm and he said: "I kept the voucher in my hand."

Kiran knew of two others who had suffered losses (but had no contact details for them). One had apparently contacted the police, but was told nothing could be done.

I contacted Ukash, which was very keen to investigate and to advise. Marketing director Paul Coxhill emailed: "Having taken some time to investigate this, it appears that there are a couple of issues to address and we would really like to work with you in highlighting these to shopkeepers.

"First, there is an important process that will help to address the issues raised. In summary, this is to ensure that all shopkeepers only issue Ukash vouchers once the consumer has handed over their cash. We would like to reiterate how important this is as it ensures that shopkeepers are not exposed to any risk of consumers leaving the store without paying.

"We will continue to reiterate this through our partner networks.

"On the occasions where this process hasn't been followed, the consumer may be able to record the voucher without paying. For example, we are aware of a small number of cases where fraudsters will illicitly film or photograph a voucher either directly or via an accomplice and then try to leave. This is extremely rare, but we want to raise it to ensure that shopkeepers do not get exposed to any risk.

"So, our main message is to always take the cash before producing the Ukash voucher."

I want to add one point of my own. Although the two retailers I spoke to said there was no sign of a camera, the 'customer' may have been armed with a secret device. I watched the debut programme of Mary Portas: Secret Shopper the other week, where she goes incognito into stores to rate their service levels. She is armed with a hidden camera... so clearly if she can do it for the good of mankind (well, for clothes shopping, so mainly womankind), then others can do it for opposite reasons.