That's for backing Heathrow's third runway, Lord Mandelson

Protesters wielding foodstuffs, paint and buckets of water have long been a messy occupational hazard for politicians venturing out in public. Eggs, flour, cakes and pies have all been used by demonstrators to make their point, but Lord Mandelson has become the first figure to fall victim to attack by green custard.

The stunned Business Secretary was left dripping from pea-coloured "slime" after he arrived at a London conference on cutting greenhouse gas emissions by industry. Within seconds of getting out of a chauffeur-driven car, an anti-Heathrow activist, Leila Deen, approached him and flung the custard contents of a coffee cup in his face. He was hustled by staff into the entrance of the Royal Society in Westminster, where he was due to address the summit, while Ms Deen walked away.

Lord Mandelson later tried to play down the incident as an "adolescent prank" and said he did not want to take it further. But the episode will have alarmed Whitehall security staff as Lord Mandelson, a former Northern Ireland secretary, is one of the most prominent members of the Government and a potential target for terrorists. Questions are bound to be asked about how Ms Deen was able to approach him unchallenged and why no police officer had been on the scene to detain her. Had the coffee cup contained acid, Lord Mandelson could have been blinded, aides said.

Last night, Scotland Yard said detectives were investigating the "circumstances surrounding the incident", but stressed police had received no complaints or made any arrests.

Ms Deen, a 29-year-old charity worker, explained she was protesting over a reported meeting between ministers and lobbyists for the British Airports Authority shortly before the Government gave the go-ahead for the expansion of Heathrow. She said: "I called to him, 'Mr Mandelson, this is for the third runway'. The only thing green about Peter Mandelson is the slime coursing through his veins. He was trying to make political capital out of this summit, but we are just not prepared to let him get away with it."

The Business Secretary emerged from the building a few minutes after the attack without his coat and wearing a tie he had borrowed from his private secretary. He said he was relieved the liquid had not been paint – and added that she had been so busy throwing the cup's contents at him that she failed to tell him what her protest was about.

Lord Mandelson added: "I don't think that anybody should over-react and, if there's a security issue, it's for the police and others. I lived with permanent round-the-clock security when I was Northern Ireland Secretary. I don't think I need to go back to that."

But John Prescott, the former deputy prime minister, who has had an egg thrown and him and a bucket of water tipped over him, was less relaxed about the attack. "What is totally unacceptable is the way the woman walked away claiming it was her right in democracy," he said on his video blog. "She should have been arrested. If it had been acid would she still be walking away? Public people shouldn't be expected to be physically assaulted with such impunity."

Senior Tory sources also said they were appalled by the incident and questioned why tougher action was not taken to protect Lord Mandelson.

Rainbow warriors: Attacks on politicians

*Cream: custard pie attack, 2008

The recently appointed Immigration minister, Phil Woolas, was speaking at a debate about the environment when a woman ran out of the audience at Manchester University and pushed a custard pie in his face. She was protesting in favour of immigration.

*Purple: flour bombs, 2004

Exchanges in the Commons between Tony Blair and the Tory leader Michael Howard were interrupted when Fathers 4 Justice campaigners hurled three purple flour bombs into the chamber. One hit Mr Blair on the back.

*Yellow: egg hurled, 2001

While campaigning in Rhyl, North Wales, in the run-up to the 2001 election, John Prescott, the Deputy Prime Minister, was hit by an egg. He instinctively lashed out at Craig Evans, lamping the farmer, who wrestled him to the ground before police intervened.

*Brown: encounter with an eclair, 2000

Nick Brown, the Agriculture minister, had a chocolate eclair smeared in his face by an activist angered by the Government's farming policy. Mr Brown insisted he was "not frightened".

*Orange: paint protest, 1995

Brian Mawhinney, the Conservative Party chairman, was flecked with orange paint by activists protesting about tougher immigration rules.