Reports that military officers will meet government officials on Monday to discuss possible US-led military action against Iran have been denied.
Iran insists its nuclear programme is for peaceful means
A Ministry of Defence spokesman said there was no truth whatsoever in the claims, made in the Sunday Telegraph.
BBC Defence Correspondent Paul Wood said US plans for a possible strike are thought to be at an advanced stage.
But US Secretary of State Condoleezza Rice said the US was "committed" to dealing with Iran diplomatically.
She told ITV1's Jonathan Dimbleby programme: "We believe that diplomacy has a chance to work but we are going to work with whomever we can, in whatever form we can, diplomatically, to try and bring the Iranians around.
"Iran is not Iraq. I know that's what's on people's minds. The circumstances are different."
She added: "However, the president of the United States doesn't take his options off the table."
UK Foreign Secretary Jack Straw, who was also interviewed on the programme, said: "We are working very hard to resolve this by diplomatic means."
He conceded UN Security Council member Russia was "anxious" about Iran.
"They are worried about the possibility of the Iranians stirring up trouble for them, but they also share our high suspicions that Iran may be using its civil nuclear capability to develop a nuclear weapon and they do not want that," he said.
The BBC's Paul Wood pointed out that many defence analysts expected that British military officials would have a wide range of contingency plans available including one for a possible US air strike on Iran.
"There is no sense that such a strike is imminent, however there is well sourced and persistent speculation that American covert activities aimed at Iran are already underway," he said.
The Sunday Telegraph reported that "a high-level meeting will take place on Monday in the Ministry of Defence at which senior defence chiefs and government officials will consider the consequences of an attack on Iran."
The newspaper stated that senior military officials would attend the meeting, along with officials from the Foreign Office and Downing Street.
In addition to denying that there would be any such meeting, the MoD said: "There will be no briefing of the prime minister and the Cabinet office in this regard, nor are there any plans for such a briefing."
Last week the five permanent members of the UN Security Council gave Iran 30 days to suspend uranium enrichment or face isolation.
According to the newspaper report, "an American-led attack, designed to destroy Iran's ability to develop a nuclear bomb, is 'inevitable' if Tehran's leaders fail to comply with United Nations demands".
Tehran insists its nuclear activities are peaceful and has rejected the council's demand.