The British Ambassador in Tehran, Richard Dalton, has been called in by the Iranian foreign ministry to receive a verbal protest over remarks made in the UK Parliament yesterday by Prime Minister Tony Blair.
Mr Blair said that the recent anti-regime demonstrations in Iran deserved Britain's support.
Protesters have also received verbal backing from US President Bush
Iranian officials have condemned the remarks as an interference in Iran's internal affairs and they have accused Mr Blair of deceiving his own public opinion over the war in neighbouring Iraq.
In a meeting which British embassy sources describe as "uncomfortable", Mr Dalton was told by
Iranian Deputy Foreign Minister Ali Ahani
that Prime Minister Blair's remarks were out of line with Britain's declared policy of non-interference in Iran's internal affairs.
They were also inconsistent, he was told, with its interest in strengthening relations between the two countries.
The Iranian side was apparently concerned that Mr Blair's remarks amounted to a shift in British policy towards Iran.
Mr Dalton sought to reassure them on that point.
He insisted that Britain remained committed to a policy of preserving and developing ties with Iran, holding dialogue on points of difference while pursuing co-operation in fields of mutual interest such as the war on narcotics.
He also said London believed that Iranians alone were responsible for their country's future.
I think people who are fighting for freedom everywhere deserve
British diplomats regard the affair as quite a serious jolt to relations between London and Tehran, which are always delicate.
It comes at a particularly sensitive moment when the possibility of another visit here by Foreign Secretary Jack Straw is under active discussion.
In separate remarks, the Iranian foreign ministry spokesman described Mr Blair's remarks as irresponsible and damaging to relations.
He broadened the counter-attack, accusing the British prime minister of trying to divert attention from his responsibility to account for the way he had deceived his own public opinion over the reasons for the war on Iraq and the continuing occupation by American and British forces.
The diplomatic tiff between Britain and Iran has made front-page headlines in some of the Iranian newspapers, with much speculation about a shift in British policy to bring it more into line with Washington's hostile attitude to Tehran.
That is a conclusion that the British embassy here is clearly at pains to counter.