David Cameron is tonight courting controversy by attending a meeting of the secretive Bilderberg Group, which comprises senior politicians, economists, business leaders and academics from Europe and North America.
The Prime Minister, who promised three years ago to lead the world's most open and transparent government, opted to join the elite gathering conducted behind closed doors at a country house hotel in Hertfordshire.
About 140 leaders from banking, finance and politics, as well as some members of royalty and aristocracy, were invited to the Bilderberg conference, which is taking place in Britain for the first time for 14 years.
The Bilderberg Group has always attracted conspiracy theories as its meetings always take place in strict secrecy and no records are released of its deliberations. It says resolutions are not proposed and no votes are taken.
Downing Street said Mr Cameron had been invited to the session as he was head of government in the conference's host country.
It said he would not be delivering a speech, but taking part in a discussion "around domestic and global issues".
The Chancellor George Osborne and Kenneth Clarke, the minister without portfolio and former Chancellor, were also on the guest-list, along with the shadow Chancellor, Ed Balls.
Other invitees included Jose Manuel Barroso, the European Commission President, Henry Kissinger, the former US Secretary of State, and Christine Lagarde, the managing director of the International Monetary Fund.
Participants, who are thought to include the billionaire heads of companies such as Google and Amazon, were greeted by protesters as they arrived in limousines with blacked-out windows for the start of the conference.
A large police operation has been mounted at its venue, the Grove Hotel near Watford, at a reported cost of £2m to the taxpayer.
Downing Street said: "The Prime Minister regularly has meetings with key ministers from other countries and with business people and others as part of his job. That doesn't mean that he is not determined to lead the most transparent government."
Since its inception in 1954, Bilderberg has held annual gatherings of 120-150 invited political leaders and experts from industry, finance, academia and the media, designed to "foster dialogue between Europe and North America".
The group describes the conference as "a forum for informal, off-the-record discussions about megatrends and the major issues facing the world" and states that the private nature of the meeting allows participants to "listen, reflect and gather insights" without being bound by the conventions of office or by pre-agreed public positions.