The criminal group behind the scam targeted the online accounts of around 100,000 taxpayers.
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An organised crime gang posing as taxpayers stole £47m (€55.8m) from Britain's tax office, after accessing more than 100,000 customer accounts, His Majesty's Revenue and Customs (HMRC) has admitted.
HMRC published details about the phishing scam, which occurred last year, on the UK government's website on Wednesday.
The UK's tax authority said it will write to affected customers, but stressed that they have not lost any money.
"This was an attempt to claim money from HMRC, not an attempt to take any money from you," HMRC said.
After accessing the accounts, the scammers managed to fraudulently claim payments from the government.
"It is unlikely that your account will have been affected. But you can check your recent account activity," HMRC told customers.
In the meantime, UK citizens are urged to stay vigilant for phishing attempts, such as suspicious emails or texts claiming to be from HMRC, as these could be linked to similar scams.
Scam kept quiet
Angela MacDonald, HMRC's deputy chief executive, told MPs at an Treasury Select Committee on Wednesday that a "lot of money" had been taken, calling the situation "very unacceptable".
She noted that HMRC had not been hacked, as it did not have data extracted from its systems.
A criminal investigation has already taken place, including in "jurisdictions outside the UK”, leading to some arrests, HMRC confirmed.
MPs reprimanded HMRC's representatives for not writing to the committee about the fraud when it occurred last year.
Committee Chair Dame Meg Hillier said she and her colleagues only heard about the scam when it was reported in the news.
The scale of the breach has raised questions about the security of personal information at a time when scammers are becoming more innovative in their methods.
MacDonald insisted that the agency’s broader efforts prevented £1.9 billion (€2.25 billion) from being taken in similar attacks last year.
"We are living in a environment where every single organisation was facing some kind of cyber threat," she said.
The deputy chief executive added that "it is a continuing piece of work for us to invest in our systems...to try to outpace the criminals".