A London woman, protesting additional runways at Heathrow Airport, threw green custard into Business Secretary Lord Mandelson's face as he arrived at the Government's low carbon economy summit Friday.

Leila Deen, an activist from the campaign group Plane Stupid, covered the politician's face in pea-green goo as he got out of his ministerial car.

He immediately went inside the building as Deen watched and then calmly walked away.

She told Sky News she was protesting reports that Lord Mandelsonmet lobbyists from Heathrow owner BAA before the Government gave the controversial go-ahead for a third runway at the London airport.

"It's green custard — I could eat this right now, it's totally harmless," Deen said about the goo.

"The only thing green about Peter Mandelson is the slime coursing through his veins. I don't want to get up early in the morning and throw custard at Peter Mandelson but I don't have a choice because democracy has failed us."

Lord Mandelson dismissed the incident, which raises questions about his personal security, as an "adolescent protest."

After a quick cleanup, he told Sky News: "She was so busy throwing what seemed like green soup or something in my face that she failed to tell me what the protest was about, but as you can see thankfully it wasn't paint and I've come through it intact."

A Scotland Yard spokesman said no complaint had been received on the matter.

Prime Minister Gordon Brownwas also due to attend the summit along with other government representatives and business leaders to discuss the "greening" of industry and opportunities for new jobs.

Click here to read more on this story from Sky News.