A senior member of Gordon Brown's government said there would be moves to replace him in September before Labour's annual conference if Labour loses the Glasgow East by-election or the party remains in the opinion poll doldrums.
The Prime Minister's fate could be sealed by a defeat on 24 July at the hands of the SNP in Glasgow, where Labour is in disarray. The selection of a Labour candidate had to be rescheduled for today after the favourite, George Ryan pulled out.
Many Labour MPs are saying privately that Mr Brown will be dealt a fatal blow if Labour does not hold the seat, which it won with a 13,500-majority at the last election.
"Gordon will have to make up his own mind on this," said one minister. "But people will encourage him, if things have not been turned around. And it will be sooner than people think.
" We can't wait for the Labour conference. There will be moves in September around the TUC conference to ask him to stand down." Labour MPs have discussed moves to collect the signatures of over 70 MPs to trigger a leadership election at the annual conference, but one Labour elder statesman said it was more likely that a delegation would urge Mr Brown to quit for the sake of the party. "Something has got to happen. The by-election could decide it," he said.
Another Labour MP, Ian Gibson, who will be defending a majority of 5,459 in Norwich North, said: "The by-election in Glasgow is crucial. If he cannot win in his own backyard, things are desperate. I think he might go voluntarily."
Some Labour MPs are resigned to defeat unless Mr Brown steps down. One despondent MP said: "I've told my general management committee not to waste funds fighting the election for me, we are going to lose. I've told them to save the money for the fightback. They are going to need it."
There are said to be only three people who could advise him to step down: his wife, Sarah, his close ally Ed Balls, and possibly the Justice Secretary, Jack Straw.
Speaking en route to a G8 summit in Hokkaido, Japan, the Prime Minister took a political risk with Labour's paymasters in the unions by rejecting demands to relax laws which curb strikes. He ruled out a change in the law allowing workers to take "sympathy action" by picketing other companies that work with firms involved in disputes.
Some unions are pressing for legislation brought in by Margaret Thatcher to outlaw secondary picketing to be scrapped. Left-wingers believe the unions should flex their muscles now they supply over 90 per cent of Labour's income.
Attacking the unions' demands, the Prime Minister told journalists: "Successful governments are those whose eyes are fixed on the future, not the past. There will be no return to the 1970s when it comes to union rights, no retreat from modernisation, and no question of reintroduction of secondary picketing rights."
He will woo moderate unions by offering to extend "family-friendly" measures, but said: "We will do nothing that puts employment and prosperity at risk."
Labour's plans in Glasgow East were thrown into disarray when George Ryan, a local councillor, pulled out, citing pressures on his family. Mr Brown is said to have phoned possible candidates who declined. Margaret Curran, a member of the Scottish Parliament, is tipped to win selection today after putting her name forward.
Some MPs want Mr Brown to appoint a senior minister as Deputy Prime Minister to aid a fightback. The favourites are Jack Straw and Alan Johnson.Reuse content