It comes as millions of people in Britain prepare to send in their tax returns online before the January 31 deadline.
The bogus email invites recipients to click on to a website which aims to collect personal and credit card information.
After submitting the requested data, the user is redirected to the real HM Revenue and Customs site.
The scam was discovered by leading internet security firm Websense which has traced the fake site to Denmark.
California-based Websense says it uses the same stylesheet and graphics as the real site, making it hard to tell the two apart.
Carl Leonard, security research manager for Europe, the Middle East and Africa, told Sky News: "Phishing scams have become increasingly sophisticated.
"They are mainly the work of highly-organised crimimal gangs who will often sell on the information they receive."
His advice to internet users is to keep their anti-virus software up to date.
That, he says, is the best way to keep the fraudsters at bay.
Anyone who does get a scam email should delete it or at the very least check it out with the organisation that purportedly sent it.
What they should never do, says Mr Leonard, is provide the information requested - usually about bank accounts or credit cards - as that will invariably be costly.
"People are more savvy about phishing than they used to be but you cannot be too careful as the malware (malicious software) is becoming increasingly subtle," he said.
A HM Revenue & Customs spokesman said: "The email is a sophisticated attempt at internet fraud.
"We are liaising closely with those agencies working to close down and prosecute those behind these scams.
"If you are in any doubt about a communication claiming to be from HMRC please contact us."
