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.

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.”