Business Secretary Lord Mandelson has had green custard thrown over him by a protester as he arrived at the launch of a low-carbon summit in London.
The protester, Leila Deen, is a member of the Plane Stupid group campaigning against a third runway at Heathrow.
The move was a "last resort" after the democratic process failed, she said.
Lord Mandelson said security was a police matter but he did not want to "go back" to the 24-hour protection he had when Northern Ireland Secretary.
The business secretary said people should not "over-react" to the incident.
Lord Mandelson stressed that he had chosen not to continue with "round the clock" protection - which he is entitled to as a former Northern Ireland secretary - and he hoped this would not now be necessary.
No complaint has been made about the incident and no action taken against Ms Deen.
But former deputy prime minister John Prescott, who threw a punch at a protester who had thrown an egg at him in 2001, said it was "totally unacceptable" that Ms Deen had been able to walk away after the incident without being arrested.
Speaking on the video sharing site You Tube, he said the protester could have thrown acid not custard and public figures had a right to go about their business without being attacked.
Lord Mandelson: "Was it guacamole or mushy peas?"
Ms Deen, 29, from Brighton, told the BBC that she had informed Lord Mandelson that the substance involved was custard before throwing it.
She said Lord Mandelson was an unelected politician and he was siding with the "interests of business" rather than the "interests of the people and the planet" in backing the new runway.
I suppose in a democracy people are entitled to have their say but I would rather people said it to my face rather than throw it
The government approved a new runway last month despite opposition by MPs, local residents and green campaigners.
She justified the "direct action" as the best way of bringing about change, claiming she and others were excluded from the democratic process.
"When democracy is failing you have to resort to any means necessary as long as it is peaceful and does not harm other human beings," she said.
She added: "Peter Mandelson is the same person who effectively bullied Ed Miliband and other members of the cabinet to accept a third runway that nobody wants, a third runway that no one was consulted on and no one is able to say no to.
"It's not right that someone like Peter Mandelson can stand up and talk about being green."
Ms Deen, who works for anti-poverty charity World Development Movement, denied indulging in "childish" stunts and hit back at criticism of her e-mailed in by BBC viewers.
"You can talk about me being a yuppie, I am no such thing, I've got a very normal background," she told BBC News, saying the protest was "a light hearted way of making a very serious point".
It's not right that someone like Peter Mandelson can stand up and talk about being green
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