The Conservative leader has said he will support the government "in doing the right thing" over Iraq.

Telling MPs that the UK is likely to be at war in the near future, Iain Duncan Smith said the country's first thoughts should now be with the troops and their families.

Speaking in a packed Commons, he said the official opposition would vote with the government.

As it emerged that two members of his frontbench - Jonathan Sayeed and John Baron had resigned over Iraq - Duncan Smith said he respected different views held by other members of parliament.

But he added that "when the government does the right thing it deserves the support of the House and the main opposition".

Addressing MPs Duncan Smith rejected suggestions that war would boost support for terrorist organisations.

But he criticised the government for failing to make sufficient preparations for dealing with the humanitarian consequences of war.

And he slammed the Liberal Democrats for saying that military action was illegal but troops should still be supported. "You cannot have it both ways, you have to make a decision and lead," he said.

While not endorsing all aspects of the government's handling of the crisis, the Tory leader warned that Saddam Hussein was a tyrant who posed a threat to the Middle East and the wider world.

"This is in the British national interest. Saddam Hussein has the means, the mentality and the motive to pose a direct threat to our national security. That is why we will be voting tonight to do the right thing by our troops and our national security."

He said the Iraqi regime had an appalling record of brutality and human rights abuses. "That is a huge and powerful argument to act."

And he warned that further delay would render the use of force impossible until the Autumn.

"The blame for further military action lies squarely in the hands of Saddam Hussein," he said.

Accepting that war would be dangerous, Duncan Smith hoped that it would at least be short lived.

He said that over 18 weeks had passed since Iraq was given a last chance under Resolution 1441.

The Tory leader urged MPs to "consider the consequences of turning back". He warned of splits in NATO, increased American isolationism and the abandonment of allies such as Spain and Australia.

"There are, I believe, matters at stake which rise above party politics.

"It is the duty of the government to act in the national interest and it is the duty of the Opposition to support it when it does so," he said

"I believe the prime minister is acting in the national interest today. This is why he is entitled to our support in doing the right thing.

"This is a critical moment for the House. If we vote to give Saddam yet another chance, the moment will pass, our concentration will falter, our energy and our focus will disperse and our nerve will fail, with all the disastrous consequences for us all.

"We cannot flunk this challenge and leave it for future generations. We cannot heap at the problems at their door and leave them to face the consequences.

"And we must not deprive our troops of the support that they fully deserve from all quarters of the House.

"We must shoulder our responsibilities and seize that moment. If we give way now, our failure will be used as a club against us in years to come."

It was time to "stand firm and act, and show that we have learned from past failures, for the fate of our security and of the wider world", Duncan Smith concluded.