Her Majesty's Revenue and Customs (HMRC) has issued a warning to anyone who has submitted a tax return for the previous financial year.
Millions of people were due to file their Self Assessment tax return by February 28.
The original January 31 deadline was extended to give extra time to people affected by the covid pandemic.
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Now, customers are being urged not to share sensitive personal information online to avoid their identities being used to commit tax fraud.
HMRC says "criminals are attempting to obtain customers’ Government Gateway logins and other personal details", enabling them to "submit bogus tax refund claims before pocketing the repayment".
Simon Cubitt, Head of Cybercrime at HMRC said: "People need to think extremely carefully before they involve themselves in an arrangement like this, because if something looks too good to be true, then it almost certainly is.
"Those who get involved risk becoming the victim of blackmail, threats of violence and wider abuse of their personal information, as criminals seek to exploit them further."
In addition to their Government Gateway logins, customers may also be asked to provide details of their bank account, passport, driving licence, address, date of birth, and National Insurance number.
Last month, HMRC made coordinated arrests of four individuals aged between 16 and 33 in Hertfordshire, Bristol, Derbyshire and Buckinghamshire as part of an investigation into suspected Self Assessment repayment fraud and money laundering offences.
Investigations are ongoing.
Anyone who suspects dishonest attempts at gaining access to personal information, or in attempting to recruit people into criminality, is urged to report it here
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