People are being warned to be on their guard against fraudulent emails telling them they were due to receive a tax refund.

HM Revenue & Customs thinks about 20,000 of the scam emails have been sent during the past week alone, as fraudsters try to cash in on people's preoccupation with tax in the run-up to the self-assessment deadline.

The emails tell people that they are due a tax refund and ask them to fill in an online form giving their bank account or credit card details so that the rebate can be paid.

The fraudsters then use the information to either empty victims' accounts or spend up to their credit limit, before passing on their details to other criminal gangs.

HMRC warned consumers not to respond to any of the emails, stressing that it only contacts people about refunds by post. It added that it was expecting there to be a "massive upsurge" in the number of so-called phishing emails immediately after the deadline as people waited to hear about genuine tax refunds.

An HMRC spokesman said: "We never use emails, telephone calls or external companies in these circumstances. We strongly urge anyone receiving such an email to send it to us for investigation before deleting it."

Last year HMRC worked with other law enforcement agencies to shut down scam networks in a number of countries, including Austria, Mexico, Korea, the US, Thailand and Japan, as well as the UK.

The warning comes just hours before the deadline for people who have to complete a self-assessment tax return to submit their form.

By noon today a record 6.2 million people had already filed their tax return online, up on the total of 5.8 million people who filed their return in this way last year, which was itself a 50% jump on the 3.8 million people who did so in 2008.

HMRC is expecting around 186,000 people to file their return as they rush to meet the deadline and avoid a £100 late payment charge. If their form has not been submitted by July 31 they are liable for another £100 penalty.

On top of this, they have to pay interest on any unpaid tax, while persistent offenders could face a penalty of up to £60 a day in certain circumstances. Every year around 10% of the 9.5 million people who have to fill in a self-assessment tax return miss the deadline. But not all of these incur the £100 penalty, as people are only fined if they owe tax.