Oxfam claims that Prince Albert, who is worth an estimated £1.35bn, received €287,000 (£194,000) in payments under the Common Agricultural Policy last year.
The subsidy was for a cereal farm of around 700 hectares. Monaco, only slightly larger than Hyde Park in London, is overwhelmingly urban and Prince Albert's farming interests are believed to be over the border in France.
The charity also said that the two biggest French recipients took €1.7m a year between them. Meanwhile, 70 per cent of small farmers received only 17 per cent of the subsidies.
"France's argument that the CAP is used primarily to support its small farmers rings hollow," said Jo Leadbeater of Oxfam. "Europe's regime of farm subsidies goes to the wealthiest landowners and destroys the livelihoods of poor farmers."
The revelations came as Britain softened its demands for cuts in European farm subsidies. The Government relaunches talks today on EU finance and the future of the UK's budget rebate from Brussels.
EU foreign ministers meet in Brussels to discuss a paper circulated by the UK, which holds the presidency of the EU and which describes the need to modernise the budget as a "long-term aspiration".
But many EU states see the UK rebate as the main stumbling block to a deal, and several ambassadors criticised the British position paper last week. One diplomat said that the document had been "destroyed" in discussions.
Britain helped block a deal in June on the EU's budget for 2007-13, saying that it would only compromise over the UK's annual €4-5bn rebate if there were agreement on reform of the Common Agricultural Policy.
The rebate was negotiated by former prime minister Margaret Thatcher in 1984 to compensate Britain for receiving relatively little from the CAP, which still accounts for around 40 per cent of EU spending.
The description of budget reform as a "long-term aspiration" comes in a three-page letter designed to restart discussions on a budget for the 25-nation bloc. The aim is to get a deal in December at a two-day summit of heads of government in Brussels.
Speaking to the European Parliament in June, Tony Blair said: "We cannot agree a new financial perspective that does not at least set out a process that leads to a more rational budget." But the language from the Government has been less precise since then, and the new document from the British presidency strikes a measured tone ahead of talks due to take place among foreign ministers today.
It says that, before putting forward detailed plans on how to solve the budget impasse, the British want broad agreement on plans outlined by the European Commission to "help the EU adjust more rapidly to the challenges of globalisation".
The measures include earmarking specific funds for research and development and creating a fund to deal with the impact of job losses.
It also highlights the need for a review clause under which member states would agree to re-examine all EU spending, including farm subsidies, during the period 2007-13.
And, in an oblique reference to the UK rebate, it says that it wants to "make proposals in line with the generally shared principle of fair treatment" as "part of an overall package covering the revenue and expenditure sides".
If Britain gets enough backing for its approach, it is expected to put forward ideas for the first two headings this month, but probably leave proposals on the most thorny issues - including the rebate - until December.
The Oxfam revelations will increase pressure for reform of the CAP, though Britain's critics point out that the country opposes measures intended to put a ceiling on payments to individual farmers.
Britain is one of relatively few countries to make payments made under the CAP public.