The foreign secretary has told the general assembly of the United Nations that the institution is stronger following the war against Iraq.

Speaking at the UN General Assembly in New York on Thursday evening, Jack Straw said the decision of the US and Britain to overthrow Saddam Hussein alone upheld the will of the international body despite huge divisions.

"What makes Iraq so important was the way it tested the role and purpose of this institution over more than a dozen years," he said.

"The international community remained in agreement throughout that the regime of Saddam Hussein posed a Chapter VII threat to international peace and security by its proliferation of weapons of mass destruction programmes, and its unparalleled defiance of the will of the United Nations.

"Yet we divided on when and what action was necessary to deal with that threat.

"Of course I acknowledge the controversy over the military action we took, and the heavy responsibilities we now bear. But I firmly believe that the decision we took was the right one."

Straw claimed "the authority of the United Nations was at stake".

"Having given Saddam Hussein's regime a 'final opportunity' to comply with the UN, what would have happened if we had simply turned away?" he asked a sceptical audience.

"Would the world be a safer place today? No. Would Iraq be a better place today? No. Would the United Nations be a stronger institution today? No.

"Saddam Hussein would have been emboldened by our failure to act, every dictator would have been encouraged to follow his example, and the authority of the United Nations would have been gravely weakened."

Standing in for the prime minister who cancelled his trip to underline his renewed commitment to domestic affairs, the foreign secretary signalled the government's determination to see the task of rebuilding Iraq through, despite calls to hand over to an international force.

"We shall stay in Iraq as long, but only as long, as it is necessary to meet our clear responsibilities; and to restore sovereignty to the Iraqi people as quickly as we can in on orderly manner," Straw said.

"I hope that we can agree a new Security Council Resolution to strengthen the UN's role in Iraq. In managing this transition, we should be guided by three central principles: first, the transfer of powers must reflect realities on the ground in Iraq, particularly the need to ensure security;

"Second, the Iraqi institutions must be sufficiently robust to take on increasing responsibilities;

"And third, the exercise of executive powers and responsibilities must be based on good governance, involving representative Iraqi authorities and coherent constitutional arrangements.

"In other words, the timetable should be driven by the needs of the Iraqi people and their capacity progressively to assume democratic control, rather than by fixing arbitrary deadlines."

And he underlined Labour's commitment to international institutions.

"The British government is profoundly committed to the ideals of the UN," Straw said.

"To us, the importance of this organisation has always been its ability to put those high ideals into effect.

"We will work wholeheartedly with the secretary general and the international community to ensure that the United Nations retains both its idealism and its effectiveness."