Tony Blair is being urged to use his influence with US President George Bush to avert military action against Iran.
Leading American former military figures said a war with the Middle Eastern state over its nuclear ambitions would be "disastrous".
And, in a letter to The Sunday Times, they called on Britain to secure a new diplomatic push to resolve tensions with the hard-line Iranian regime.
"An attack on Iran would have disastrous consequences for security in the region, coalition forces in Iraq and would further exacerbate regional and global tensions," they said.
"The current crisis must be resolved through diplomacy. The British government has a vital role to play in securing a renewed diplomatic push and making it clear that it will oppose any recourse to military force."
The letter comes ahead of a major report to be published on Monday in which think tanks, unions, non-governmental organisations and faith groups will warn of the consequences of military action.
The co-ordinated calls to avert military action come amid signs that Washington is becoming impatient with Iran, which has refused to ditch its nuclear programme.
The letter is signed by Lieutenant General Robert Gard, a former military assistant to the US Defence Secretary; General Joseph Hoar, a former commander in chief; and Vice Admiral Jack Shanahan, a former director of the Centre for Defence Information.
It coincides with demands from faith leaders in the Independent on Sunday for diplomacy to be pursued further.
Rabbi Tony Bayfield, the head of the Movement for Reform Judaism, Lord Bishop Harries of the Church of England and Senior Imam Sadjid of Brighton, said there was "no justification" international law for military force against Iran. "There is still time to talk and we urge everyone to use it and pray for success," they say.
