A climate change activist threw a cup of green "slime" over Business Secretary Lord Mandelson today as he arrived at the launch of the Government's low carbon economy summit.

Leila Deen from campaign group Plane Stupid said she was protesting at reports that Lord Mandelson met lobbyists from Heathrow owner BAA before the Government gave the controversial go-ahead for a third runway at the London airport.

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The incident happened a few seconds after Lord Mandelson got out of his ministerial car to attend the summit in central London.

Ms Deen, 29, threw a cup full of what she described as "green slime" over the minister, who tried to duck when he realised what was happening.

Ms Deen, who calmly walked away after the incident, said: "The only thing green about Peter Mandelson is the slime coursing through his veins."

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

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."

Ms Deen questioned the Business Secretary's attendance at the summit.

"That Mandelson is trying to make political capital out of climate change just days after reports that he met with BAA's top lobbyists to push through the third runway is an insult to my generation," she said.

Ms Deen accused Lord Mandelson of only representing business interests, adding that he was an unelected Government representative.

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Lord Mandelson quickly walked into the building where the summit is being held after the incident and emerged a few minutes later, minus his coat.

Prime Minister Gordon Brown was 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.

Plane Stupid has launched a series of high profile stunts in recent years against the Government's environmental policies.

Ms Deen was one of three Plane Stupid protesters who sounded horns during a speech at an environmental conference in London by Transport Secretary Geoff Hoon earlier this week.

She and the other demonstrators left after being warned they would be thrown out if they interrupted the speech again.

At one point they sounded horns when Mr Hoon reached the part of his speech when he said that proper debate rather than "stupid stunts" was the way to address the aviation-and-the-environment issue.

Ms Deen added later: "We know that Mandelson is best buddies with BAA's top lobbyist Roland Rudd, 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."

A Scotland Yard spokesman said no arrests had been made and an investigation would only be launched if a complaint was received.

Ms Deen told the Press Association later that the substance she threw at the minister was custard, which had green colouring added to it.

"Peter Mandelson is trying to say he doesn't know what it was about, but as I threw it I said 'This is for the third runway'.

"He knows he is in bed with business, working against the interests of the climate and the people of this country.

"He was trying to make political capital out of this summit but we are just not prepared to let him get away with it."