Discussions have taken place between British and American officials over the use of UK bases in the event of military action against Iran, Downing Street indicated.
No 10 refused to be drawn on reports that the Government had rebuffed a plea by the US for its forces to be allowed to use bases in Britain as well as in Cyprus and on Ascension Island in the Atlantic and Diego Garcia in the Indian Ocean to support the military build-up in the Gulf.
However, a spokeswoman confirmed that the issue of the bases - which have been used by the US in the past, including during the invasion of Iraq - had been raised in discussions between officials.
"Contingency planning is something which we do as a matter of routine," the spokeswoman said.
"Obviously we are working closely, for example with the United States, as we have done in the past, regarding the use of UK bases. We routinely speak to our counterparts in the United States. We don't get into details of those discussions, but we have in the past co-operated on the use of UK bases."
The Guardian reported that US diplomats have been sounding out British officials about the use of the bases - which in previous conflicts have been used by US bombers to fly long-range missions. They were said to have received a cool response from the British, who cited legal advice from the office of the Attorney General, Dominic Grieve, that a pre-emptive military strike against Iran would be illegal under international law.
The paper quoted a senior Whitehall source as saying: "It is explicit. The Government has been using this to push back against the Americans."
The US was said to have been "surprised" by the reluctance of UK ministers to provide assurances of this kind of "upfront assistance". "They'd expect resistance from senior Liberal Democrats, but it's the Tories as well. That has come as a bit of a surprise," a source told the paper.
No 10 refused to comment on Mr Grieve's legal advice. "We don't comment on the legal advice that the Attorney General provides," the spokeswoman said.
She said the Government was still committed to resolving the stand-off over Iran's nuclear programme through a combination of sanctions and diplomatic engagement. She reiterated David Cameron's recent warning that military intervention was not the right course "at this time" but that "no option is off the table".