Protesters have scaled a crane opposite the Houses of Parliament to call for a referendum on the European treaty, in the third such stunt seen in the last week.

Days after green demonstrators mounted the roof of a British Airways plane at Heathrow and a separate group staged a rooftop protest at the House of Commons, two men climbed hundreds of feet late last night and were still there this morning.

Their banners, saying "Referendum Now" and "Give Us Our Vote", were aimed at forcing the Prime Minister to call for a referendum before adopting the European treaty, which they claim is merely a rebranding of the constitution.

Today's stunt comes ahead of Wednesday’s House of Commons vote on a Conservative proposal to hold a referendum.

However it appeared to be having little impact. As demonstrations are permitted in Parliament Square, which is not inside the Parliamentary estate, police say the pair are doing nothing illegal by protesting there and their only offence appeared to be trespassing on a crane.

A security source said that police officers were not concerned about the demonstration. "They'll come down when they get cold," the source said.

Philip Webster, Political Editor of The Times, said headline-grabbing protests like that seen today – along with the stunts of recent days – have little impact on public opinion.

"It is almost counter-productive, because it raises all kinds of wider issues on MPs' minds, such as security," he said.

"After the demonstration last week on the roof, there was only one MP who actually backed the protesters, and that was John McDonnell, whose constituency covers Heathrow.

"When protests like that seen today take place, the story ends up not being about Heathrow, for example, but about the security breach, or the stunt.

"What happened in Parliament Square this morning will have absolutely no impact on Wednesday's debate about a referendum."

One protester, calling himself Simon, who like his counterpart was wearing a fluorescent jacket and helmet, said today: "We want the Government to give the people of Britain a referendum like they promised they would. It’s a very simple request.

"It’s a question of democracy. All we are asking for is a vote on whether to ratify this treaty or not. It seems Gordon Brown doesn’t trust us enough to let us have a vote." They claimed to represent a group calling itself We Are Change.

Asked whether he backed their protest, the Conservative leader said he supported the protesters’ message but not their methods.

“No-one should break the law; that’s not right. But on the issue of a referendum on the Lisbon Treaty, the constitutional treaty, of course we should have one," David Cameron told Sky News.

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