Tony Blair is confident of a second UN resolution approving military action against Iraq - despite strong opposition from France, Russia and Germany.
School children protest outside Downing Street
The prime minister's spokesman said there was an "inescapable logic" to UN resolution 1441 which warned of serious consequences if Iraq refused to disarm.
The UK would not have talked about a second resolution - seen as key to winning public support for a war - if it was not serious about getting it, the spokesman said.
France, Germany and Russia said earlier on Wednesday they would not allow a UN resolution that authorises war against Iraq.
During the weekly prime minister's question time, Mr Blair said all world leaders he had spoken to agreed Saddam Hussein was failing to comply with UN demands to disarm.
War could be avoided if the Iraqi regime fully complied with the order to disclose and destroy its weapons of mass destruction or if Saddam Hussein left the country.
Asked if a majority of the Security Council backing military action would be enough, Mr Blair said that the effect on the UN of not standing firm would be "disastrous".
Even if Iraq scrapped all its Al Samoud missiles, it would still be in material breach of resolution 1441 because it had not disclosed and destroy chemical and biological weapons, he said.
200,000 US troops in region
106,000 more are en route or on alert
30,000 UK troops in position
UK Tornado and Harrier aircraft deployed
Sixth US aircraft carrier group heading to Gulf
Further public splits between Western nations seem inevitable on Friday, when chief weapons inspector Hans Blix is scheduled to present his latest report on Iraqi disarmament.
Ahead of that report, Dr Blix said on Wednesday that Iraq was making "greater efforts" and the scrapping of the missiles was "real disarmament".
Earlier, French Foreign Minister Dominquie de Villepin said: "We will not allow the passage of a planned resolution which
would authorise the use of force.
"Russia and France as permanent
security council members will fully assume all their responsibiltites."
Thousands of children took time off school to protest against possible war on Wednesday.
Whitehall was closed for a time as more than hundred children demonstrated at the gates of Downing Street.
Before his appearance in Parliament, Mr Blair met Russian foreign minister Igor Ivanov for 50 minutes at Downing Street.
Mr Blair's official spokesman said that although Russia "comes at this from a different direction" they agreed on the need for Iraq to comply with existing UN resolutions.
On Tuesday Mr Ivanov told BBC News Online that Russia might veto a second Security Council resolution about military action.
A Russian abstention in the vote on the US/UK second resolution would increase pressure on France as it decided whether to be the only permanent security council member to veto the resolution.
But Mr Ivanov told the BBC's Talking Point: "Abstaining is not a position Russia can take, we have to take a clear position and we are for a political solution."
The latest talks come as a Mori poll suggests conditional support in the UK for British troops being involved in a war against Iraq is picking up.
Support for British involvement in any action fell from 71% last September to 61% in January, but has now risen to 75%, the Mori survey suggested.
But the figures are only in the event of clear evidence that Iraq is concealing weapons of mass destruction and of the UN Security Council passing a second resolution backing war.
Without these only 24% would be willing for Britain to join the US attack, and two-thirds would be opposed.
In September, about 70% of Britons opposed an attack without explicit UN backing, a figure which rose to 77% in January.