PRINCE Charles accepted a suitcase containing €1 million in cash from a former Qatari prime minister, it has emerged.
The Sunday Times reported it was one of three cash donations from Sheikh Hamad bin Jassim totalling €3m handed over between 2011 and 2015.
Clarence House has responded saying all the money was passed immediately to one of the prince's charities and all correct processes were followed.
There is no suggestion the payments were illegal.
Campaign group Republic says it raises "serious ethical questions" and has now demanded full disclosure from the Prince of Wales.
Northern Ireland secretary Brandon Lewis has, meanwhile, insisted the Prince has done nothing wrong.
It has been reported once of the donations was handed over in a holdall at Clarence House. On another occasion, the cash was apparently passed on in Fortnum and Mason carrier bags.
In a statement, Clarence House said: "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."
Republic will be writing to Prince Charles, the UK Government, MPs and the Charity Commission, calling on all relevant authorities to investigate and disclose the nature of Charles's financial dealings with Sheikh Hammad.
Speaking for Republic today, Graham Smith said: "This story is shocking. Prince Charles met Sheikh Hamad in private, with no officials present and with no disclosure of the meeting in the court circular."
"The Sheikh then made three payments directly and personally to Prince Charles, totalling €3m, which Charles gave to a charity he set up to pursue his pet projects and help run one of his estates.
"Sheikh Hamad faces serious accusations over human rights and has significant financial and other interests here in the UK.
"Given that Prince Charles has direct access to the British prime minister and all government ministers, as well as all cabinet papers, this raises serious ethical questions about what the sheikh expected in return.
"Was he able to use Charles as a channel for influencing the UK Government or to gain inside knowledge of government business?
"This also adds to the serious questions about Prince Charles's judgement, following from other accusations of cash-for-access and cash-for-honours.
"The Prince of Wales’s Charitable Fund also needs to answer questions about its acceptance of this money and its independence from Charles."
The funds were received by the Prince of Wales's Charitable Fund, which aims to "transform lives and build sustainable communities" by awarding grants to good causes in fields such as conservation, education, health and social inclusion.
The fund told the Sunday Times that its trustees had concluded the donor was legitimate and that its auditors had signed off on the donation.
Donations to Prince Charles's charities have come under scrutiny in recent months following allegations that one of them offered a Saudi donor help to secure a UK honour and citizenship.
The Metropolitan Police said earlier this year that it is investigating the claims about the Prince's Foundation under the Honours (Prevention of Abuses) Act 1925.
Lewis said there was no evidence Prince Charles had done anything wrong by accepting the donations from Sheikh Hamad bin Jassim.
Lewis said: “All of these donations into these charities go through proper due processes.
“It’s right that they do that in the same way that political donations and donations to any charity should go through a proper process and they're supporting those issues that so many people care about.
“I think it is clear that this went through, this money went through proper due process as it rightly should.”
