If Peter Mandelson hoped to make a splash with a speech on the government's low carbon energy initiatives yesterday, he certainly managed to do so, though perhaps not quite as he intended: an anti-airport protester threw a cupful of green liquid in the business secretary's face as he arrived outside the Royal Society in central London, and walked away with apparent impunity.

News cameras showed Lord Mandelson genially approaching a young woman and then ducking as he was splattered with what was later identified as custard with colouring thrown from a coffee cup. As Mandelson hurried inside, the woman explained what she had done and then, clearly surprised not to be detained, walked away. She identified herself as Leila Deen, 29, a supporter of the Plane Stupid group campaigning against airport expansion.

Deen told reporters she had been motivated by reports that the minister had met lobbyists for the British Airports Authority, which wants a third runway at Heathrow, a few days before the government endorsed the plan.

She said: "The only thing green about Peter Mandelson is the slime coursing through his veins. That he is trying to make political capital out of climate change ... is an insult to my generation. He is unelected and only represents business interests."

Deen said Mandelson was an anti-green influence on the government. "He came back and suddenly we've got a third runway that nobody wants. It's quite clear whose side he's on. He's from this generation of people who don't care what kind of legacy they're leaving."

Mandelson later told journalists he had dispensed with special security after he left the Northern Ireland Office. The Home Office declined to discuss whether ministerial security would now be reviewed. Mandelson described the incident as an "adolescent protest". Detectives were investigating, but Scotland Yard said no arrests had been made and no one had made a complaint.

The business secretary emerged from the Royal Society in a clean shirt and with a silk tie donated by one of his civil servants. He told journalists: "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 was not paint and I have come through it intact."

Deen said: "He's been actively pushing a high-carbon future through the third runway. I didn't want to let him stand up and talk about that, so last night I decided to make some custard, colour it green, and show how slimy I think he is."

Asked if she thought her action was non-violent, she said: "Yeah, absolutely," as it had been "simply harmless custard". She said. "The police came up to me and said that they wanted to check I wasn't going to try to enter parliament. They said they couldn't comment on what I'd done, but they hoped I had a lovely day."

Mandelson said later that he had never been afraid for his safety. "Thankfully it was sort of, I think, organic and non-toxic. 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. I don't think that anybody should overreact. I'm not afraid for my security as long as I'm doing my job, doing it conscientiously. I guess I should be grateful to the protester for helping us to put this very important subject on the map."

Gordon Brown also laughed it off, saying: "If anybody doubted the greening of Peter Mandelson and his willingness to take the green agenda on his shoulders, we've seen it in practice this morning."

But the former deputy prime minister, John Prescott, said in his video blog: "What is totally unacceptable is the way the woman walked away claiming it was her right in democracy. She should have been arrested. If it had been acid would she still be walking away? We had better get a grip of this situation and, frankly, public people shouldn't be expected to be physically assaulted with such impunity."

Deen's mother, Sheila, herself a long-standing member of Greenpeace, said at her home in Brighton: "I'm proud she's got the courage of her convictions and she's prepared to take direct action for injustice. It's not easy, to know you run the risk of being arrested. When Leila usually makes custard it's quite lumpy, but this looked pretty smooth."