Major retailers are being urged to share details of an iTunes scam with staff so they can help prevent customers from being conned.
HM Revenue and Customs has written to the chief executives of major UK retailers as part of efforts to raise awareness of the scam, which it said is tricking vulnerable and elderly people out of thousands of pounds.
It said there have been "reassuring" reports of supermarket staff helping to keep their customers safe.
The scam involves fraudsters cold calling people pretending to be HMRC members of staff.
They tell them they owe large amounts of tax which they can only pay off through Apple's iTunes vouchers.
Those targeted are told to go to a local shop, buy these vouchers and read out the redemption code to the scammer.
The fraudsters then sell on the codes or buy high-value products, all at the victims' expense.
Figures from Action Fraud show that there have been over 1,500 reports of the scam since 2016. The vast majority of victims are over 65, suffering an average financial loss of £1,150 each although for some the loss has been significantly higher.
Following a recent awareness campaign, several retail workers across the UK have warned customers buying hundreds of pounds-worth of iTunes vouchers that they may be about to fall victim to the scammers, HMRC said.
Angela MacDonald, HMRC's director general of customer service, said: "It's really reassuring to see reports of supermarket staff, off their own back, taking action to keep customers safe. Raising public awareness is the best safeguard against this vicious scam.
"Supermarket staff are often the last line of defence against these fraudsters. That's why I've written to the chief executives of major UK retailers to urge them to make their staff aware of this scam so they can help protect unsuspecting customers."
HMRC said that, as well as raising awareness, its action to shut down scamming operations includes identifying and initiating the takedown of website links being used by criminals.