Tens of thousands of fraudulent e-mails have been sent out ahead of Sunday's tax return deadline, officials say.
Recipients are told they are due a tax refund and asked to fill in an online form with bank or credit card details.
Victims have their accounts emptied or card limits used, and risk having the details sold to other criminal gangs.
Revenue & Customs (HMRC) has warned taxpayers not to respond to the "phishing" e-mails, as it informs customers of a refund by post only.
An HMRC spokesman said: "We never use e-mails, telephone calls or external companies in these circumstances.
"We strongly urge anyone receiving such an e-mail to send it to us for investigation before deleting it."
The Revenue said at least 20,000 fake e-mails had been sent out in the past week alone.
HMRC is expecting a massive upsurge in such correspondence following the 31 January deadline when many people will be waiting to hear about genuine tax refunds.
Investigations by HMRC alongside law enforcement agencies have shut down scammers in the UK and countries such as Austria, Mexico, South Korea, the US, Thailand and Japan in the last year.
About 5.8 million people filed their self-assessment tax return online last year, up from 3.8 million the previous year.
Self-assessment is required primarily from the self-employed or those who have income from several sources.
Anyone who completes the details online after 31 January, or who filed on paper after 31 October, could face a fine of £100.