Self-assessment taxpayers are coming under "sustained attack" from gangs of organised fraudsters, HM Revenue & Customs (HMRC) has told The Independent on Sunday.

Thousands of taxpayers are routinely receiving letters and emails from fraudsters posing as the HMRC in an attempt to trick them into revealing financial details about themselves, such as credit card and current account numbers – a practice known as phishing. The information is used to drain money from victims' accounts and even steal their identity.

"It is very disturbing that fraudsters are using our good name to get money out of people," an HMRC spokesman says. "The letters and emails claim that the taxpayer is either owed tax or has outstanding tax to pay, and ask for card or account details.

"People need to be on their guard. The fraudsters are very sophisticated. Some bogus letters come on headed paper and there are even telephone numbers which work and are answered by fraudsters."

The HMRC website currently lists nine different types of fraudulent emails and letters doing the rounds; last December the same website listed just one. There has also been a surge in activity ahead of the deadline for completion of self-assessment tax returns on 30 September.

"That the UK self-assessment payer is being targeted right now is no surprise to me," says Anthony Riem, partner and fraud expert at law firm PCB. "Fraudsters play on any weaknesses. They know people are expecting to get communications from HMRC at this time of year and could be fooled by the offer of a tax rebate into being incautious and handing over information they shouldn't."

Similar attempts have been made to defraud taxpayers in the US, which operates its own type of self-assessment. "Generally with fraud, what happens in the US tends to happen here a little while later," adds Mr Riem.

Bogus letters and emails vary in sophistication. Some are easy to spot, thanks to mis-spellings and fraudsters using obviously fake names for the HMRC, such as the "Inland Revenue Board" or "Inland Revenue & Customs Board".

Another giveaway is when the letter or email asks for an outstanding tax bill to be paid by credit card, as the HMRC does not accept payment this way.

But the scams are getting more sophisticated.

"[Communications] can be very convincing-looking," says Chas Roy-Chowdhury, head of taxation at the Association of Chartered Certified Accountants. "Some clone the logo and symbols of HMRC."

He adds, however: "UK tax authorities are aware of the threat. They seem, for once, to be ahead of the curve on this one."

Nevertheless, Mr Riem says the Government should do more to alert taxpayers to the threat: "What about an advertising campaign to let people know that these fraudulent emails and letters are out there?"

HMRC, which spends £4.2m a year alerting taxpayers to the self-assessment deadlines, says it is taking action. Interpol has been called in to track down the fraudsters.

"We do take this very seriously and we urge anyone who receives a letter or email to contact their local tax office if they are unsure of anything," the HMRC spokesman says.

"These fraudsters are sending thousands of these emails and letters out each day and they only need a few to bite to make it worth their while."

Around four and a half million taxpayers are expected to file their self-assessment form by 30 September. People who miss this deadline will have to calculate their own tax or use the HMRC's online filing system. They have until 31 January to do this or face an automatic £100 fine.