The Bilderberg group, an elite coterie of Western thinkers and power-brokers, has been accused of fixing the fate of the world behind closed doors. As the organisation marks its 50th anniversary, rumours are more rife than ever.

Given its reputation as perhaps the most powerful organisation in the world, the Bilderberg group doesn't go a bundle on its switchboard operations.

Telephone inquiries are met with an impersonal female voice - the Dutch equivalent of the BT Callminder woman - reciting back the number and inviting callers to "leave a message after the tone".

Anyone who accidentally dialled the number would probably think they had stumbled on just another residential answer machine.

But behind this ultra-modest façade lies one of the most controversial and hotly-debated alliances of our times.

On Thursday the Bilderberg group marks its 50th anniversary with the start of its yearly meeting.

For four days some of the West's chief political movers, business leaders, bankers, industrialists and strategic thinkers will hunker down in a five-star hotel in northern Italy to talk about global issues.

What sets Bilderberg apart from other high-powered get-togethers, such as the annual World Economic Forum (WEF), is its mystique.

Not a word of what is said at Bilderberg meetings can be breathed outside. No reporters are invited in and while confidential minutes of meetings are taken, names are not noted.

The shadowy aura extends further - the anonymous answerphone message, for example; the fact that conference venues are kept secret. The group, which includes luminaries such as Henry Kissinger and former UK chancellor Kenneth Clarke, does not even have a website.  

In the void created by such aloofness, an extraordinary conspiracy theory has grown up around the group that alleges the fate of the world is largely decided by Bilderberg.

In Yugoslavia, leading Serbs have blamed Bilderberg for triggering the war which led to the downfall of Slobodan Milosevic. The Oklahoma City bomber Timothy McVeigh, the London nail-bomber David Copeland and Osama Bin Laden are all said to have bought into the theory that Bilderberg pulls the strings with which national governments dance.

And while hardline right-wingers and libertarians accuse Bilderberg of being a liberal Zionist plot, leftists such as activist Tony Gosling are equally critical.

A former journalist, Mr Gosling runs a campaign against the group from his home in Bristol, UK.

"My main problem is the secrecy. When so many people with so much power get together in one place I think we are owed an explanation of what is going on.

Mr Gosling seizes on a quote from Will Hutton, the British economist and a former Bilderberg delegate, who likened it to the annual WEF gathering where "the consensus established is the backdrop against which policy is made worldwide".

"One of the first places I heard about the determination of US forces to attack Iraq was from leaks that came out of the 2002 Bilderberg meeting," says Mr Gosling.

But "privacy, rather than secrecy", is key to such a meeting says Financial Times journalist Martin Wolf, who has been invited several times in a non-reporting role.

"The idea that such meetings cannot be held in private is fundamentally totalitarian," he says. "It's not an executive body; no decisions are taken there."

As an up-and-coming statesmen in the 1950s, Denis Healey, who went on to become a Labour chancellor, was one of the four founding members of Bilderberg (which was named after the hotel in Holland where the first meeting was held in 1954).

His response to claims that Bilderberg exerts a shadowy hand on the global tiller is met with characteristic bluntness. "Crap!"

"There's absolutely nothing in it. We never sought to reach a consensus on the big issues at Bilderberg. It's simply a place for discussion," says Lord Healey.

Formed in the spirit of post-war trans-Atlantic co-operation, the idea behind Bilderberg was that future wars could be prevented by bringing power-brokers together in an informal setting away from prying eyes.

"Bilderberg is the most useful international group I ever attended. The confidentiality enabled people to speak honestly without fear of repercussions.

"In my experience the most useful meetings are those when one is free to speak openly and honestly. It's not unusual at all. Cabinet meetings in all countries are held behind closed doors and the minutes are not published."

That activists have seized on Bilderberg is no surprise to Alasdair Spark, an expert in conspiracy theories.

"The idea that a shadowy clique is running the world is nothing new. For hundreds of years people have believed the world is governed by a cabal of Jews.