The government has reportedly told the United States that it will not allow American troops to use the British military in a decision that could have an impact on a potential pre-emptive strike against Iran.

It had previously been believed that Diego Garcia, the British-governed island in the Indian Ocean, could be drawn into an Israel-Iran conflict.

But the Guardian reported on Friday that Britain had used secret legal advice to suggest any pre-emptive strike could breach international law.

The newspaper said US diplomats had lobbied for troops to use Diego Garcia, as well as British bases in Cyprus and Ascension Island in the Atlantic Ocean, during their planning for a possible conflict in the Gulf, but had been rebuffed.

In July, Foreign Office Minister Lord Howell had indicated that the opposite could be the case.

During a Lords debate he was asked whether the government would deny permission for Diego Garcia to be used for logistical support if the US backed Israel during military action.

Lord Howell replied: “The answer is obvious. If Israel was to take military action, the United States would be involved and if the United States was involved, we would be involved.

"This would be a triggering of global action and Diego Garcia would therefore obviously be dragged into it.”

The Guardian claimed a British military delegation flew to Tampa, Florida, to discuss contingency plans with US counterparts during the summer.