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Fraudsters are targeting people with scam e-mails in the run-up to the self-assessment tax returns deadline.
HM Revenue and Customs (HMRC) said the e-mails suggested the recipients were due for a tax refund and went on to ask for bank or credit card details.
Victims who provided these details faced the threat of their accounts being emptied of funds.
The deadline for filing genuine returns is 31 January. Anyone who files late faces a £100 penalty.
HMRC said it was also aware of a growing number of telephone scams in which fraudsters posed as tax officials arranging rebate payments.
But it was particularly concerned about the e-mail scam, which thousands of people had received and many more were set to receive in the coming weeks.
"This is the most sophisticated and prolific phishing scam that we have encountered," said Lesley Strathie, HMRC chief executive.
"We only ever contact customers who are due a refund in writing by post. We never use e-mails, telephone calls or external companies in these circumstances.
"I would strongly encourage anyone receiving such an e-mail to send it to us for investigation."