The British government has said it had received a request from Washington for troops to take part in a possible Iraqi military campaign.
But Defence Secretary Geoff Hoon stressed that did not mean war against Iraqi President Saddam Hussein was inevitable.
Asked if the United States had asked for a contribution of forces to a potential invasion, Mr Hoon told a news conference: "We have had a request for forces from the US, but can I emphasise no decision has been taken on that."
He said full details would be given to parliament next Monday, when it will hold a debate on Iraq.
Many members of Mr Tony Blair's Labour Party are furious at the prospect of Britain following President George W. Bush into an attack on Baghdad.
About 150 Labour lawmakers have signed up to a motion expressing "deep unease" about a war.
Iraq has promised to give a complete account of its arms programme under a new UN resolution. Mr Bush and Mr Blair have demanded Saddam shed his weapons of mass destruction or face being disarmed by force.
The two leaders will meet at a NATO summit in Prague later today.
Mr Bush said today he would call on NATO allies to help him disarm Saddam should the Iraqi leader refuse to give up the deadly arsenal Washington believes he has concealed.
Britain was the only country other than the United States to contribute a full armoured division to the 1991 Gulf War, but troops covering for striking firefighters now will stretch the armed forces to the limit.
Mr Hoon could spell out next week plans to call up reserve troops. Such a move requires a parliamentary order to be utilised.