LONDON — A protester threw green custard in the face of British business minister Peter Mandelson on Friday, in a stunt to highlight opposition to a new runway at London’s Heathrow airport.

The former EU trade commissioner was arriving for a London summit on carbon strategy when protester Leila Deen approached him and hurled a large cupful of custard straight in Mandelson’s face, from close range.

Mandelson bent over, turned away and hurried inside, while the protester picked up papers she had dropped and calmly strode off.

“Mandelson is trying to make political capital out of climate change,” said serial campaigner Deen, a member of the group Plane Stupid, which has staged several protests over plans to build a third runway at Heathrow, in west London. “The only thing green about Peter Mandelson is the slime coursing through his veins.

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“We can’t let the ‘Prince of Darkness’ cast his shadow over west London” — a reference to Mandelson’s nickname earned for his media spinning skills.

The incident appeared to raise security questions, since Deen, 29, approached Mandelson unchecked and walked off freely afterwards.

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Mandelson, who returned to the British government in October, seemed unfazed by the stunt, saying he did not fear for his security and people should not over-react.

“It could have been nastier,” the 55-year-old told reporters later — minus his splattered coat.

“Whilst I’m prepared to take my fair share of the green revolution on to my shoulders, I’m less keen on having it on my face.

“In a sense I guess I should be grateful to the protester for helping us to put this very important subject on the map.” However, he added: “I would rather people said it to my face than threw it in my face.”

A Scotland Yard spokesman said: “Police are investigating circumstances surrounding an incident in Carlton House Terrace, SW1, at about 8:00am today. There have been no arrests. No complaint has been received.”

Former Deputy Prime Minister John Prescott — who famously punched a protester in the face after being pelted with an egg in 2001 — voiced anger and said that Deen should have been arrested.

“For those moments when you receive that kind of assault, you don’t know what it is. He couldn’t have known whether that was acid being thrown in his face,” he said on his video blog.

“What is totally unacceptable is the way the woman walked away, claiming that it was her right in a democracy. She should have been arrested… If it would have been acid, would she still be walking away?”

“Public people shouldn’t be expected to be physically assaulted with such impunity,” he added. “It could have been much more serious. Let’s do something before it is.”

Charity worker Deen told BBC television: “It’s a light-hearted way of making a very serious point… Peter Mandelson had it coming. Everything he touches turns to sleaze.”

Bookmaker William Hill is taking bets on which popular dessert would next be hurled at a British government minister, with spotted dick, a steamed suet pudding containing currants, the 3/1 favorite.