The taxman is warning of the huge risk from scams as it has received almost 900,000 reports of people impersonating HM Revenue and Customs.
Crooks are calling, texting and emailing people, pretending to be from the tax office, hoping to lure them in to hand over personal details so they can use them to carry out fraud.
More than 100,000 of the suspicious contact reports were phone scams, while over 620,000 were about bogus tax rebates.
The most common techniques used by fraudsters are calling taxpayers offering a fake tax refund and asking for bank details.
Or pretending to be HMRC by texting or emailing a link which takes victims to a false page, where their bank details and money will be stolen.
And threatening people with arrest or imprisonment if a bogus tax bill is not paid immediately.
In the run up to the self-assessment deadline HMRC is warning the millions of people who fill out these forms to be on their guard as it is expecting crooks to be out in force.
Gareth Shaw, head of money at Which?, said: “The number of people targeted by HMRC scams is staggering and the problem is only likely to get worse as the self-assessment deadline looms.
"Sophisticated tactics like number spoofing see innocent people losing life-changing sums everyday.”
HMRC and banks will never contact customers asking for their PIN, password or bank details.
Sarah Coles, personal finance analyst at investment firm Hargreaves Lansdown, said: “Millions of us are already facing the misery of completing a self assessment tax return.
"Haven’t we suffered enough? Apparently not - tax return season is open season for scammers, who are keen to part us from even more of our hard-earned cash.”
