He is known as the “Prince of Darkness”, the ultimate spin doctor who has been at the heart of the New Labour project. And today Lord Mandelson came face to face with the reality of the British government’s pro-aviation expansion policy – literally.

Mandelson was hit in the face with a cup of green slime by an activist from the group, Plane Stupid, as he arrived to attend an “low-carbon” energy summit in central London in his thirsty gas-guzzling Jaguar.

News cameras showed Mandelson receiving a large proportion of green-coloured custard in the face as he tried to duck out of the way.  The activist who “pied” Mandelson was Leila Deen, 29, from Plane Stupid, who shouted “This is for the third runway”.

Deen told reporters she had been motivated by reports that Mandelson had held a series of meetings with lobbyists for British Airports Authority, the owner of Heathrow a few days before the government endorsed the plan.

“We know that Mandelson is best buddies with BAA’s top lobbyist Roland Rudd,” said Deen, “and reports suggest it was him who bullied Energy Secretary Ed Miliband into accepting a third runway. We can’t let the Prince of Darkness cast his shadow over west London. The only thing green about Peter Mandelson is the slime coursing through his veins. He is unelected and only represents business interests.”

Deen was referring to new “favours for friends” revelations in the Daily Mail newspaper last weekend that had reported how “Peter Mandelson is at the centre of a ‘favours for friends’ row over the controversial proposals to expand Heathrow.”

The paper had reported the “extraordinary access the Business Secretary’s close acquaintance Roland Rudd  –  who represents airport operator BAA  –  had to the top tiers of Government. Mr Rudd, the City’s most powerful PR, or his company Finsbury Ltd met with ministers at least five times in ten days in the run-up to Labour’s unpopular decision to go ahead with plans for a third runway, and once a few months earlier.”

Also attending the “low carbon” conference later today will be British Prime Minister Gordon Brown who will tell the carbon summit that moving Britain to a low carbon economy will create 400,000 new “green” jobs over the next eight years.

Mr Brown will call for an international “green new deal” to boost the environmental sector and help lift the global economy out of recession.

The first part of any green new deal should be the scrapping of London’s third airport.