Iran strike 'would be disastrous'
Washington has so far refused to rule out military action if Iran does not halt its nuclear activities.
The US and its Western allies suspect Iran of using its nuclear programme as a cover to produce nuclear weapons, a claim denied by Tehran.
Recent criticism by President George Bush of alleged Iranian support for insurgency in Iraq has increased concerns that his administration is contemplating an attack.
The pressure group, called Crisis Action, brings together trade unions, charities and Christian and Muslim organisations. They include the Amicus, Unison and GMB unions, the Oxford Research Group, the Foreign Policy Centre, Pax Christi, the Muslim Council of Britain and the Muslim Parliament. The group's document is entitled " Time to Talk".
It says: "The consequences of any possible future military action could be wholly counter-productive as well as highly dangerous. Diplomatic solutions to the Iranian nuclear issue must be pursued resolutely."
The call by Crisis Action has been backed up by a letter to the Sunday Times from three former senior US military commanders - Generals Robert G Gard Jr and Joseph P Hoar and Vice Admiral Jack Shanahan. General Hoar is a former commander in chief of US Central Command, which covers the Middle East.
Their statement says: "An attack on Iran would have disastrous consequences for security in the region, collation forces in Iraq and would further exacerbate regional and global tensions. The current crisis must be resolved through diplomacy.
"There is time available to talk."