The Charity Commission is examining whether it needs to carry out a review of donations received by a charity of the Prince of Wales.
The Sunday Times reported that millions of euros in cash were handed over by a former prime minister of Qatar.
"We will review the information to determine whether there is any role for the commission in this matter," said the charities regulator.
Prince Charles's representatives say "all correct processes were followed".
The Charity Commission will examine whether it needs to take any action over the claimed donation of three million euros from Sheikh Hamad bin Jassim to Prince Charles.
Some of the money was reported to have been in cash in a suitcase and in carrier bags from the upmarket shop Fortnum and Mason.
But it's understood that donations from the Sheikh were also made by bank draft and there could be a challenge over details of the amounts and how payments were handed over.
The donation was for the Prince of Wales's Charitable Fund, which provides grants for conservation, environment, education and social inclusion projects.
The donations from Sheikh Hamad bin Jassim were reported to have been made between 2011 and 2015 - but more recent financial accounts from the Charity Commission, between 2017 and 2021, show the charity had an annual income between £10m and £15m.
The charity regulations allow donations to be made in cash and for donors to remain anonymous.
But charities would have to carry out due diligence to ensure that the funds were from a legitimate source.
"Charitable donations received from Sheikh Hamad bin Jassim were passed immediately to one of the prince's charities, who carried out the appropriate governance and have assured us that all the correct processes were followed," said a statement from Clarence House.
Earlier this year the Charity Commission ran a formal inquiry into the charities of Viateschlav Kantor, a billionaire oligarch sanctioned by the UK over Russia's invasion of Ukraine.
Mr Kantor's charity had made , the Prince's Foundation.
Charity records show £600,000 was paid to the foundation over two years, as part of a pledge for £3m, that had been intended to be paid in stages until 2028.
The Prince's Foundation had said it wouldn't comment on individual donations, but donors would have been subject to a process of due diligence, based on what was known at the time.