Gordon Brown is the second most unpopular Prime Minister in modern British history, an opinion poll showed on Friday, but unseating him would do little to help his Labour party's fortunes.

In a week when Foreign Secretary David Miliband wrote an article calling for change in the party, a move interpreted in the media as laying the ground for a leadership challenge, the poll was further crushing news for Mr Brown.

The Prime Minister, who succeeded Tony Blair without an election in June last year, can now count on just 15 per cent support, according to the YouGov poll, the lowest rating ever for a leader aside from John Major in 1995.

A weakening economy, with house prices falling and credit conditions tight, has compounded Mr Brown's misfortunes.

"I think the dice have been rolled," John Curtice, a professor of government at Strathclyde University and a noted commentator on British politics, said.

"It's pretty clear that an important game is going to get played out in September. There might be some sort of attempt to get him to go, but whether it will be successful remains to be seen. It's game on, put it like that."

Britain's Parliament is now in recess for the summer, meaning some of the momentum calling for Mr Brown to go is likely to die down for the coming weeks. Mr Miliband, the architect of the latest ructions, has gone on holiday to Spain.

But if the chatter of discontent continues into late September, when Labour holds its party conference, Mr Brown's leadership credentials could again be under the spotlight.

The problem for Labour is that few senior party figures have a high national profile. Polls show that even if Mr Brown were to be replaced by a more youthful figure such as Mr Miliband, the party would still not beat the Conservatives in an election.

The party also cannot afford to be seen to chop and change leader at every turn - if Mr Brown were to be ousted it would be the third Prime Minister in 18 months - without calling an election, something the Conservatives want.

Those factors alone make a leadership challenge unlikely, political analysts say. But in the past week, since Mr Miliband effectively stuck his head above the parapet, the odds on such a bold move being mounted have shortened.

It would not be a straightforward task, however. At least 70 Labour members of parliament would have to back a challenge - a high threshold - and they would only do it if they were certain a new leader could revive the party's prospects before the next election, which has to be held by May 2010.

"Mr Brown's rivals now have a fine calculation to make," the Conservative-leaning Telegraph newspaper, which published the YouGov poll, wrote in an editorial.

"They have a leader with historically low ratings but an electorate that believes removing him will solve nothing. And won't discarding leaders like dirty socks only push Labour even deeper into public contempt?"

Advisers close to Mr Brown dismiss the leadership chatter as "midsummer madness". They are focused on rebuilding confidence in Mr Brown over the coming months, they say, and looking for the economy to start to recover next year, buoying the government.

Mr Miliband himself has tried to damp down the furore caused by his article, without explicitly endorsing Mr Brown.

"Can Gordon lead us into the next election and win?" he asked rhetorically at a news conference this week. "Yes, I'm absolutely sure of that."