In rejecting the informal request, British officials cited legal advice indicating a pre-emptive strike on Iran could violate international law, The Guardian reported Thursday.
The Guardian said it was told U.S. diplomats have sought use of British bases on Cyprus, and asked permission to fly from U.S. bases on Ascension Island and Diego Garcia atoll, both of which are British territories.
British ministers have reacted coolly to the overtures so far, The Guardian said. Officials point up advice drafted by the attorney general's office that has been circulated to the prime minister, the Foreign Office and the Defense Ministry.
The draft makes clear Iran currently doesn't pose "a clear and present threat," The Guardian reported, so providing assistance to forces that may be involved in a pre-emptive strike would clearly violate international law.
"[Britain] would be in breach of international law if it facilitated what amounted to a pre-emptive strike on Iran," a senior government official said. "It is explicit. The government has been using this to push back against the Americans."
Sources told the publication the United States has not submitted a formal request.
Britain is operating on the premise it would become involved only if conflict has begun, officials told the newspaper. Israel has indicated a willingness to launch a military strike against Iran's nuclear facilities to help prevent the country from developing a nuclear weapon.
"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 [British] would not be told beforehand," one official said. "In some respects, the U.K. government would prefer it that way."
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