Fraudsters are targeting taxpayers with scam emails purporting to come from HM Revenue and Customs (HMRC) in the run up to the 31 January self assessment tax deadline.
Small business owners and the self-employed are being sent fake "phishing" emails to try to trick them into handing over their bank or credit card details, in what HMRC is describing as "the most sophisticated and prolific scam" it has dealt with.
HMRC said it was receiving around 500 of these emails each day, which had been forwarded by consumers.
Since April 2008, HMRC has received over 11,000 reports of fraudulent repayment emails being send to its customers, but the problem has grown significantly this month.
HMRC urged people that receive an email to send it to the department for investigation.
The tax officials said HMRC does not use emails, telephone calls or external companies to notify customers who are due a refund. HMRC only ever notifies people in writing by post.