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."