Britain has rebuffed US pleas to use military bases in the UK to support the build-up of forces in the Gulf, citing secret legal advice which states that any pre-emptive strike on Iran could be in breach of international law.
The Guardian has been told that US diplomats have also lobbied for the use of British bases in Cyprus, and for permission to fly from US bases on Ascension Island in the Atlantic and Diego Garcia in the Indian Ocean, both of which are British territories.
The US approaches are part of contingency planning over the nuclear standoff with Tehran, but British ministers have so far reacted coolly. They have pointed US officials to legal advice drafted by the attorney general's office which has been circulated to Downing Street, the Foreign Office and the Ministry of Defence.
This makes clear that Iran, which has consistently denied it has plans to develop a nuclear weapon, does not currently represent "a clear and present threat". Providing assistance to forces that could be involved in a pre-emptive strike would be a clear breach of international law, it states.
"The UK would be in breach of international law if it facilitated what amounted to a pre-emptive strike on Iran," said a senior Whitehall source. "It is explicit. The government has been using this to push back against the Americans."
Sources said the US had yet to make a formal request to the British government, and that they did not believe an acceleration towards conflict was imminent or more likely. The discussions so far had been to scope out the British position, they said.
"But I think the US has been surprised that ministers have been reluctant to provide assurances about this kind of upfront assistance," said one source. "They'd expect resistance from senior Liberal Democrats, but it's Tories as well. That has come as a bit of a surprise."
The situation reflects the lack of appetite within Whitehall for the UK to be drawn into any conflict, though the Royal Navy has a large presence in the Gulf in case the ongoing diplomatic efforts fail.
The navy has up to 10 ships in the region, including a nuclear-powered submarine. Its counter-mine vessels are on permanent rotation to help ensure that the strategically important shipping lanes through the Strait of Hormuz remain open.
The Guardian has been told that a British military delegation with a strong navy contingent flew to US Central Command headquarters in Tampa, Florida, earlier this summer to run through a range of contingency plans with US planners.
The UK, however, has assumed that it would only become involved once a conflict had already begun, and has been reluctant to commit overt support to Washington in the buildup to any military action.
"It is quite likely that if the Israelis decided to attack Iran, or the Americans felt they had to do it for the Israelis or in support of them, the UK would not be told beforehand," said the source. "In some respects, the UK government would prefer it that way."
British and US diplomats insisted that the two countries regarded a diplomatic solution as the priority. But this depends on the White House being able to restrain Israel, which is nervous that Iran's underground uranium enrichment plant will soon make its nuclear programme immune to any outside attempts to stop it.
Israel has a less developed strike capability and its window for action against Iran will close much more quickly than that of the US, explained another official. "The key to holding back Israel is Israeli confidence that the US will deal with Iran when the moment is right."
With diplomatic efforts stalled by the US presidential election campaign, a new push to resolve the crisis will begin in late November or December.
Six global powers will spearhead a drive which is likely to involve an offer to lift some of the sanctions that have crippled Iran's economy in return for Tehran limiting its stockpile of enriched uranium.
The countries involved are the US, the UK, France, Germany, Russia and China. Iran will be represented by its chief negotiator, Saeed Jalili.
A Foreign Office spokesman said: "As we continue to make clear, the government does not believe military action against Iran is the right course of action at this time, although no option is off the table. We believe that the twin-track approach of pressure through sanctions, which are having an impact, and engagement with Iran is the best way to resolve the nuclear issue. We are not going to speculate about scenarios in which military action would be legal. That would depend on the circumstances at the time."
The Foreign Office said it would not disclose whether the attorney general's advice has been sought on any specific issue.
A US state department official said: "The US and the UK co-ordinate on all kinds of subjects all the time, on a huge range of issues. We never speak on the record about these types of conversations."
The Israeli prime minister, Binyamin Netanyahu, warned at the UN general assembly last month that Iran's nuclear programme would reach Israel's "red line" by "next spring, at most by next summer", implying that Israel might then take military action in an attempt to destroy nuclear sites and set back the programme.
That red line, which Netanyahu illustrated at the UN with a marker pen on a picture of a bomb, is defined by Iranian progress in making uranium enriched to 20%, which would be much easier than uranium enriched to 5% to turn into weapons-grade material, should Iran's supreme leader, Ayatollah Ali Khamenei, take the strategic decision to abandon Iran's observance of the nuclear non-proliferation treaty and try to make a weapon. Tehran insists it has no such intention.
In August, the most senior US military officer, General Martin Dempsey, distanced himself from any Israeli plan to bomb Iran. He said such an attack would "clearly delay but probably not destroy Iran's nuclear programme".
He added: "I don't want to be complicit if they [Israel] choose to do it."

UK holds talks with US on possible Iran scenarios

By DAVID STRINGER | Associated Press – 37 mins ago

LONDON (AP) — Britain is involved in military contingency planning with the United States over Iran and other potential flashpoints in the Middle East, officials said Friday — but they insisted the talks are not a prelude to a pre-emptive strike against Tehran's nuclear program.
Prime Minister David Cameron's office confirmed that routine military planning is being carried out with the U.S. and other allies on a range of scenarios, including on the potential use by American forces of British bases, some of which can act as staging posts for missions to the Middle East.
The Guardian newspaper reported in Friday's editions that the U.S. had asked Britain to use its bases in Cyprus, and British territory in the Atlantic and Indian Ocean, to help build up forces in the Gulf. It reported that move was regarded as a contingency in case of the need for strikes to halt Tehran's nuclear program.
"Contingency planning is something which we do as a matter of routine. Obviously we are working closely, for example with the United States, as we have done in the past, regarding the use of U.K. bases," a spokeswoman for Cameron told reporters, on condition of anonymity in line with policy.
"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 cooperated on the use of U.K. bases," she said. The U.S. military used British bases in the buildup to the 2003 invasion of Iraq.
Britain's Foreign Office said that the U.K. was involved in "prudent" contingency planning with its allies, including the U.S.
"The government does not believe that military action against Iran is the right option at this time, but we are not taking any option off the table," Cameron's spokeswoman said.
She insisted that Britain remained committed to a policy of imposing ever tighter sanctions against Iran, while also seeking to engage Tehran in talks aimed at ensuring the country has access to civilian nuclear power but abandons its alleged pursuit of an atomic weapon. Tehran insists it is not developing nuclear arms.
Britain's government declined to comment on The Guardian's claim that U.K. attorney general Dominic Grieve had issued legal advice cautioning that any involvement in a pre-emptive strike on Iran — including cooperating on bases — would violate international law.
The newspaper, citing unnamed government sources, reported that in his advice, the government's chief legal advisor had stated Iran did not constitute "a clear and present threat."
Israel has been especially wary of Iran's nuclear plans, and has warned it would use military force to prevent the Islamic Republic from obtaining atomic weapons.
In a speech earlier this month, Cameron said he had warned Israel's Prime Minister Benjamin Netanyahu that any strike on Iran would risk bolstering support for the regime in Tehran. He insisted that international measures, including a European ban on Iranian oil imports, were crippling Iran's economy.
"We need the courage to give these sanctions time to work," Cameron said.