A environmental campaigner who threw green custard over Business Secretary Peter Mandelson was given a police caution today.

Leila Deen, 29, threw the liquid over Mandelson as he got out of a car to attend a meeting to promote low-carbon industries early last month.

Deen, from the group Plane Stupid which is opposed to plans to expand London's Heathrow Airport, said her protest was to highlight what she said was the government's hypocritical policies on climate change.

"This is a light-hearted way of making a very serious point," she said at the time.

Police said Deen had been given an official caution following advice from the Crown Prosecution Service.

Mandelson made light of the incident after he had cleaned himself up.

"I don't quite know what point she was making because 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," he said.

"But as you can see it wasn't paint and I've come through it intact."

The government approved plans in January for the expansion of Heathrow airport by adding a third runway and another terminal, angering environmentalists.

The protest was one a series of high profile demonstrations carried out by Plane Stupid.

Members of the group have also cut through a perimeter fence at Aberdeen airport and disrupted flights, while last year they scaled the roof of the House of Commons and draped banners down the side of the building.