A crucial Commons vote on Iraq is taking place after Tony Blair urged MPs to back his stance on disarming Saddam Hussein.
MPs are filing through the voting lobbies to cast their verdict on the prime minister's polict on Iraq.
As Foreign Secretary Jack Straw wound up the debate after almost eight hours there was disruption in the Commons chamber as protestors shouted from the public gallery.
Earlier, in one of the most important speeches of his career, the prime minister said the only people who would suffer if Saddam is not tackled will be the Iraqi people.
And he warned that the Iraqi leader would be strengthened "beyond measure" if the world fails to tackle his weapons of mass destruction.
"Who will celebrate and who will weep if we pull our troops back now?" he said.
MPs will vote on the government's policy on Iraq at 22:00GMT on Tuesday.
Mr Blair warned that retreat would send a dangerous message to other "tyrants", while the Iraqi people would be left in "pitiless terror".
165 Labour MPs must rebel before Mr Blair has to rely on Tory votes
245 Labour MPs must rebel for Mr Blair to lose a vote on Iraq, even with Tory support
The sums explained
He said: "I will not be party to such a course. This is not the time to falter.
"This is the time for this House to give a lead, to show we will stand up for what we know to be right."
Some commentators saw his comments as a hint he would quit rather than pull UK forces out of the Gulf.
The prime minister was speaking as two more ministers resigned over his stance on Iraq.
Mr Blair is facing a major backbench rebellion over plans to take military action in Iraq.
Home Office Minister John Denham and Health Minister Lord Hunt quit the government on Tuesday along with two ministerial aides.
The departures followed in the wake of Robin Cook's decision to resign from the cabinet on Monday.
But Mr Blair was boosted by International Development Secretary Clare Short's decision to stay in the cabinet, despite saying she was still "very critical" of the handling of the crisis.
Mr Denham said he had resigned because he believed it was crucial to have international consensus for pre-emptive action.
A YouGov opinion poll for ITV News on Tuesday suggested that Mr Blair is winning over public option for a war on Iraq.
A total of 50% said they supported military action, while 42% said they were
US President George Bush has given Saddam Hussein and his sons 48 hours from 0100 GMT on Tuesday to leave Iraq or face invasion - a demand later rejected by the Iraqi leadership.
The White House described Saddam Hussein's move as his "final mistake".
President Bush's spokesman, Ari Fleischer, said allied troops were going to enter Iraq "no matter what".
In the Commons, Mr Blair said that if the UK pulled out of possible military action it would give America "the biggest impulse to unilateralism you can imagine".
Setting out the stakes for MPs ahead of Tuesday's key Commons vote he argued they would determine the path of international politics for the next generation.
"This is a tough choice indeed but it is
also a stark one: to stand British troops down ... or hold firm to the course we
have set," he said.
Mr Blair, who met Labour backbenchers privately before the debate, decried Iraqi "lies, deception and obstruction" about weapons of mass destruction.
It was "palpably absurd" to believe that Iraq had voluntarily disarmed after United Nations weapons inspectors left in 1998, he argued.
The prime minister urged MPs to "show that we will stand up for what we know to be right
... that we will confront the tyrants and terrorists who put our lives at risk
... that we have the courage to do the right thing."
Rebel MPs have drawn up an amendment to the government's motion stating there is no moral justification for war without a new resolution.
The last debate on the Iraq crisis saw 122 Labour MPs voting against the government and rebels hope more will join then this time.
Moving the amendment, former Labour Defence Minister Peter Kilfoyle called war "illegal, immoral and illogical".
But Conservative leader Iain Duncan Smith backed the government's stance.
Saddam Hussein had "the means, mentality and motive" to
threaten Britain's national security, he said.
But Liberal Democrat leader Charles Kennedy said: "We do not believe that there
is a case for war that has been established in the absence of a second
That did not mean, however, that his party would not give troops put into action "full moral support", he added.
The Tories saw a further three resignations over Iraq after the departure as a whip of John Randall last week.
Shadow environment minister Jonathan Sayeed, shadow home affairs minister Humfrey Malins and shadow health minister John Baron all left their posts on Tuesday.