The British government says it reserves the right to attack Iraq without prior U.N. approval if Iraq defies a Security Council disarmament resolution.
The British parliament took up the question of Iraq in a special debate Monday.
Prime Minister Tony Blair said his government is still working for a peaceful solution through the United Nations in the showdown over Iraq's weapons program.
But he says he cannot accept any restrictions on the use of British military force if Iraqi President Saddam Hussein defies a U.N. disarmament resolution.
"We want to keep some freedom of maneuver and also keep maximum pressure on Saddam," he said. "He has got to know that unless he gets rid of weapons of mass destruction, then, if he doesn't do it peacefully through the weapons inspectors, he will have it done by force."
Iraq denies it possesses weapons of mass destruction, and says it will cooperate with the U.N. weapons inspections.
Mr. Blair's foreign secretary, Jack Straw, told parliament the British armed forces are getting ready to fight in case the U.N. initiative collapses.
"It is crucial that we make proper preparations. As the United Nations process moves forward, so should our preparedness for military action in the event that this process fails," he said.
Mr. Straw tried to allay the fears of some in parliament that Britain is blindly following the United States into a war in Iraq.
He said Britain would likely seek a second United Nations resolution to authorize force if Iraq does not comply with the disarmament initiative.
And he said parliament should vote on sending British troops to war, though he said that to preserve the element of surprise, a vote might not occur until after the shooting starts.
The defense secretary, Geoff Hoon, had been expected to tell parliament about a call-up of military reservists. Mr. Straw said there would be no such statement, but he did not explain why the plans had changed.