US Defence Secretary Donald Rumsfeld has sparked diplomatic confusion by suggesting America has alternative plans if the UK decides not to go to war with Iraq.
The US is preparing for war without its closest ally
The remarks - later retracted - caused shock and surprise in Downing Street, which chose to focus on continuing efforts to secure a new United Nations resolution.
But rebel Labour backbenchers have seized on the remarks and are expected to urge Tony Blair at Commons question time on Wednesday to withdraw British military involvement.
The diplomatic flurry came as the prime minister said he was willing to work "night and day" to secure enough common ground among UN security council members for agreement on a new resolution.
Mr Rumsfeld told reporters the UK's role was "unclear" because of Tony Blair's difficulties in convincing a rebellious Parliament of the need for military action.
They (the US) can do it without us and give Tony Blair the
chance to get out of the hole if he wishes
Graham Allen MP
Asked if he meant the US would go to war without its closest ally, he added: "That is an issue that the president will be addressing in
the days ahead, one would assume."
A Downing Street spokeswoman insisted: "This has not changed anything. We are still working to get a second
resolution. We are not at this stage (war) yet.
"But there has been complete cooperation throughout between the United
Kingdom and United States on the military planning."
Within the hour, Mr Rumsfeld tried to clarify his comments with a statement saying he had "no doubt" in "a significant military contribution from the United Kingdom."
But Liberal Democrat foreign affairs spokesman Menzies Campbell told BBC's Newsnight that Mr Rumsfeld's comments appeared to "devalue Britain's military contribution and hence its political influence".
Labour MP Graham Allen said: "The cat is out of the bag. They can do it without us and give Tony Blair the
chance to get out of the hole if he wishes."
The prime minister's frantic international negotiations continue on Wednesday at dinner with German Chancellor Gerhard Schroeder at Downing Street.
After talks with the Portuguese and Romanian prime ministers on Tuesday, Mr Blair warned that Saddam Hussein will be "let off the hook" if France or Russia uses a veto over a further UN resolution.
UK diplomats at the UN have proposed a series of tests they say Baghdad should fulfil within a set time to prove that it is ready to hand over its weapons.
The proposals are part of an attempt to win wider support for a new UN resolution that gives the Iraqi leader a deadline to disarm before war.
Mr Blair hopes the plan will break the UN deadlock and ease mounting political pressure at home following an attack on his strategy by Clare Short, the international development secretary.
On Tuesday, six undecided UN members - Cameroon, Angola, Chile, Guinea, Mexico and Pakistan - suggested a 45-day deadline for Iraq to disarm.
But this was rejected by America, which is insisting that a UN vote on war against Iraq will happen this week.
Meanwhile, the Ministry of Defence has denied claims by the Public Accounts Committee that it has not learned enough from mistakes made during a large desert exercise in Oman two years ago.
More than 20,000 troops were put through their paces amid criticism about the equipment used.