The prime minister has been told by supporters and critics that it is "crunch time" on a possible decision to go to war with Iraq.

The warning came as Tony Blair set off for an emergency summit which will decide the next moves in the crisis.

Aides described the meeting on Sunday with US president George Bush and Spanish premier Jose Maria Aznar as a "last push" for consensus in the United Nations.

The UK prime minister spent the weekend engaged in a frantic round of phone calls with leaders of other security council members urging them to back a compromise resolution.

But he faces continued opposition to his plans from Labour backbenchers.

Former culture secretary Chris Smith said MPs would have to decide this week whether they would back the government.

"I would say to them, look, this is actually the crunch time, this is the moment when as a country, as a parliament, we say, 'is it right to go to war now, this week?'"

Union leader Bill Morris warned that the bond of trust between prime minister and people would suffer massive damage if the country went to war.

"I think it will be a very, very, very long time before that trust can be rebuilt, and I think that will be the greatest loss to the prime minister and the party if that bond of trust is broken as a result of this war," he told the GMTV Sunday Programme.

"It seems very difficult for anyone to begin to give support to an act, which the UN secretary general believes to be unlawful."

But the government's position was defended by Welsh secretary Peter Hain, who said that opinion was shifting towards the prime minister.

"Talking to party activists, talking to MPs, a feeling of really this is a crunch time, and whereas over the past weeks people were very, very critical, people now realise that because of the French veto, we can't really walk away from this situation.

"On the question of party membership, and tearing up party membership cards, there's been a lot of extravagance, exaggeration about this," Hain said.

Tony Blair met with a succession of senior cabinet ministers in Downing Street on Saturday.

Deputy prime minister John Prescott, Labour chairman John Reid and other senior ministers visited Number 10 on Saturday as the prime minister sought their backing before heading to the summit.

Speaking after his meeting with Blair, David Blunkett said that hopes for a peaceful end to the crisis had been "thrown into complete chaos" by French pledges to veto any second resolution.

"I think the prime minister and our foreign secretary have done everything in their power to ensure that we found a peaceful way through," the home secretary said.

Labour's left signalled over the weekend they were far from convinced about the need for military action.

Also over the weekend it emerged that left wing MPs were stepping up their calls for the government to set out the basis of its legal case for military action.

John McDonnell, acting on behalf of the Socialist Campaign Group, wrote to Commons Speaker Michael Martin asking him to ensure that the advice is made available to parliament.