The Yes Men are Andy Bichlbaum and Mike Bonanno, two enigmatic activists who use the internet and media to lampoon corporations and politicians.

Although it is unclear which of them fronted the elaborate hoax that tricked the BBC into broadcasting a fake apology for the Bhopal disaster - the so-called spokesman looked like Bichlbaum - the stunt carries all their hallmarks.

They have attacked Dow Chemical before, along with President Bush and the World Trade Organisation in a career of international spoofs and wind-ups. It is not clear how far the group extends beyond the duo.

Typically Bichlbaum and Bonanno set up lookalike websites, filling them with content that parodies the ideals and goals of their victim. They also distribute bogus pamphlets and impersonate officials and spokesmen.

Bichlbaum and Bonanno call it "identity correction" - exposing what they claim to be injustices by powerful bodies against the global majority.

During this year's US elections many voters were duped into signing up for fake petitions accepting a nuclear dump in their neighbourhood or to having their children engage in any war with North Korea.

The pair spent three years impersonating World Trade Organisation spokesmen at business conferences, trying to shock unsuspecting audiences with ironic plans such as recycling meat for the Third World.

Bichelbaum, 40, said: "Yes men are people who agree with the boss. They are constantly nodding to advance themselves. We agree extra wholeheartedly with people, not to advance ourselves but to make clear what they are thinking and communicate it with the rest of the world."

The resulting material from the WTO spoof has been used to produce a book and a film that chronicles their campaign.

Their antics started in 1993 when Bonnano, 36, swapped voice boxes in talking Barbie and GI Joe dolls so that Barbies would snarl "Dead men tell no lies" and GI Joes would say "Maths is too hard". The trick dolls were returned to San Francisco toy shops and sold to baffled kids over Christmas.

Funding for their capers comes from arts grants and income from lectures and donations. The pair claim to have over 30,000 people on their mailing list and their website invites anyone engaged in "identity correction" to join them.