Nick Clegg, a candidate for the Liberal Democrat leadership, has warned against Britain "sleepwalking" into a conflict with Iran and called on Gordon Brown to rule out any prospect of supporting military action by the United States against the country.
The Prime Minister said this week he will "rule nothing out" while seeking a diplomatic resolution to the demands by the United Nations for Iran to suspend its enrichment of uranium.
But in a letter to Mr Brown, Mr Clegg expressed concern that given the Prime Minister's comments it was possible that the government could repeat the strategic mistakes that led to the invasion of Iraq.
In the letter Mr Clegg writes: "The rumble of war with Iraq started with similar sabre-rattling from President Bush. The war with Iraq has put back the cause of Middle East peace, failed to make Iraq more stable and has made it more difficult for Britain to argue for the rule of international law."
The Prime Minister has made it clear that he will push for tougher sanctions against Iran's oil and gas industry as well as tightening the financial screws on Iranian banks if the report on Iran's nuclear programme published today by Mohamed ElBaradei, the head of the International Atomic Energy Agency (IAEA), shows that Tehran is still defying the international community. However, he has refused to go as far as Jack Straw, when he was foreign secretary, in saying that support for military action against Iran would be "inconceivable".
Mr Clegg said it was vital to show absolute commitment to international law, but it appeared that Mr Brown was ready to give "a blank cheque" to President Bush to pursue an aggressive policy "with no sense of independent British foreign policy priorities".
He warned: "As President Bush nears the end of his term in office, it is essential that his administration is left in no doubt that a last-minute dash towards unilateral military action will not be supported by Britain.
"Such action would risk another major conflagration in the region and in any event is widely viewed by experts as implausible on military grounds alone."
Mr Clegg said it was "nonsense" to suggest all options should be left open to maximise leverage on Iran. Threats of military action risked strengthening the position of the hardliners in Iran, he said.
Mr Brown has said he believes the sanctions are beginning to work, but there are fears in Whitehall that ruling out military action would ease the pressure on Mahmoud Ahmadinejad, the Iranian President, ahead of today's IAEA report. They say cracks are beginning to show in his regime. Ali Larijani, the former top security official in Iran, resigned last month in the midst of tensions with his President over the negotiations with the UN.
Mr Brown has offered a compromise proposal of an international nuclear energy bank which could supply Iran with enriched uranium for its civil nuclear power programme, without risking highly enriched, weapons-grade material being transferred to the country.
The shadow Foreign Secretary, William Hague, warned, in another open letter, against delaying tougher sanctions if today's report shows Iran is in breach of commitments under the Nuclear Non-Proliferation Treaty, following reports that Tehran was seeking to stall talks with Javier Solana, the EU's senior diplomat.
"As a result of this inaction, there is a grave risk that our diplomatic efforts to constrain Iran's nuclear programme may be overtaken by events," said Mr Hague. He said it was estimated that Iran had 2,000 centrifuges in its underground enrichment plant and it would only need 3,000 to produce enough uranium for a weapon in six to 12 months.