Banksy’s friend smokes spliff in public gallery as Nick Clegg addresses the Commons

A street-artist friend of Banksy was able to smoke a cannabis joint in the House of Commons during Prime Minister's Questions.

"Cartrain", 18, smuggled the drugs past security officers and lit up in the public gallery as Deputy Prime Minister Nick Clegg — standing in for David Cameron — was taking questions from MPs yesterday lunchtime.

Cartrain and a friend were finally hauled out by police after he stood up and shouted "decriminalise cannabis", although the glass security screen meant few in the chamber below could hear his protest and the session was not interrupted. The Leytonstone teenager, who once removed part of Damien Hirst's Tate Britain installation Pharmacy in a separate protest, was later released with a warning.

He said: "The security was tighter than at an airport — there were about 20 cops searching people, metal detectors, the works — but I couldn't believe how easy it was.

"I lit up right in front of Nick Clegg. I must have taken a good four or five pulls on the spliff before the guards realised what I was doing, and then I went quietly. They said they would put me in a cell under Big Ben, which I said sounds amazing, like the most expensive hotel in London, but then they decided to let me go when they realised I was quite keen to be arrested."

His police stop and account form, signed by an officer of the Met's SO17 Palace of Westminster division, records: "Stopped in strangers gallery after seen smoking cannabis."

A spokesman for the House of Commons said: "Two people were removed from the public gallery of the Commons today for inappropriate behaviour. They were handed over to the police, cautioned and given advice about drugs."

Cartrain's previous stunts include sneaking a fake exhibit into the British Museum.

Last year artists including Banksy rallied to support him after he was arrested for taking a packet of pencils — valued at £500,000 — from Hirst's exhibition after the former Young British Artist's representatives threatened to sue Cartrain for making collages including photos of Hirst's work.