The UK government has rejected requests by the US to use British bases to support an eventual attack on Iran.

The Guardian reported on Thursday that the US also lobbied for the use of British bases in Cyprus, Diego Garcia and Ascension Island, but British authorities said that legal advice suggested that any pre-emptive strike on Iran could be illegal.

Officials said a secret document, drafted by the attorney general’s office, says that Iran does not represent “a clear and present threat”, which means, assisting forces that are attacking Iran would be a clear breach of international law.

“The UK would be in breach of international law if it facilitated what amounted to a pre-emptive strike on Iran," a senior Whitehall source told The Guardian.

"It is explicit. The government has been using this to push back against the Americans."

There has been no official request by the US, and it is thought that the informal request were to gauge Britain’s position on the issue.

“But I think the US has been surprised that ministers have been reluctant to provide assurances about this kind of upfront assistance," said another source to The Guardian.

"They'd expect resistance from senior Liberal Democrats, but it's Tories as well. That has come as a bit of a surprise."

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."