Diplomatic relations between Britain and the United States over Iran are under increasing strain after Gordon Brown's special security adviser warned that American claims about Tehran's military capability should be taken 'with a pinch of salt'.

As a new conservative campaign group with links to the White House prepares to make the case that Iran is a direct threat to the US, Patrick Mercer urged scepticism towards any US justification for strikes against the country.

Mercer, the former shadow homeland security spokesman, who visited the Iranian capital recently, said: 'There is increasing concern about the apparent evidence that America is preparing about Iranian military involvement.'

Mercer, who last month accepted a post as an adviser to the Brown government, said: 'All that I heard when I was in Iran was British authorities saying "be careful about what you hear from America". I'm not saying for one moment that it is necessarily wrong, but it's got to be taken with a pinch of salt. Is it American rhetoric, propaganda or fact?'

Brown Aid Plays Down US Talk Of Iran Threat