Britain said on Friday it was opposed to a military strike on Iran "at this moment" over its disputed nuclear program, arguing sanctions were having an effect and diplomacy should be given time.
The comments followed a report by Britain's Guardian newspaper which said Britain had rebuffed US plans to use its bases to support the build-up of troops in the Gulf, due to legal advice warning that a pre-emptive strike would be illegal.
The legal advice says Iran currently does not represent a "clear and present threat", according to the Guardian, which cited unnamed sources.
"The government does not believe military action against Iran is the right course of action at this moment, though no option is off the table," UK Prime Minister David Cameron's spokeswoman told reporters, declining to comment on the legal advice.
"We want to see the sanctions, which are starting to have some impact, working, and also engaging with Iran," she said.
The Guardian said Britain had not received a formal US request to use its bases for a military build-up.
Cameron and Western diplomats believe harsh sanctions imposed on Iran by the West are beginning to weaken Tehran's resolve and to stoke public discontent, and that military action would reverse the trend and rally Iranians to the government.
Israel and the West believe Iran is trying to achieve nuclear weapons capability. Tehran says its program is for purely civilian, energy purposes.
Years of diplomacy and sanctions have failed to resolve the dispute, raising fears of Israeli military action against its arch foe and of a new Middle East war.
Talks between the West and Iran could take place after the Nov. 6 United States presidential election, following three inconclusive rounds this year.