Britain today ordered the expulsion of two Iranian diplomats from London in a tit-for-tat move that signalled a deepening rift between the two countries.

Prime Minister Gordon Brown told MPs two UK diplomats had been ordered to leave their Tehran embassy yesterday and he had decided on a swift response.

The dependants of British diplomats are already being evacuated from the Iranian capital after the UK was singled out for fostering unrest on its streets.

Mr Brown said: "It is with regret that I inform the House that Iran yesterday took the unjustified step of expelling two British diplomats over allegations which are absolutely without foundation.

"In response to that action, we informed the Iranian ambassador earlier today that we would expel two Iranian diplomats from their embassy in London.

"I am disappointed that Iran has placed us in this position but we will continue to seek good relations with Iran and to call for the regime to respect the human rights and democratic freedoms of the Iranian people."

Conservative leader David Cameron said the expulsion of the British diplomats was "clearly not acceptable" and backed the retaliatory measure.

Both the British and the Iranian diplomats have been given a week to pack their bags.

Downing Street said the Iranian ambassador was summoned to the Foreign Office this lunchtime to be told by Permanent Secretary Peter Ricketts that two diplomats would have to leave.

The Iranians had ordered the expulsion of the two British officials for "activities inconsistent with their diplomatic status" - usually the code for spying.

The Foreign Office described the Iranian allegations as "baseless".

A spokeswoman said the Iranians appeared to be trying to blame Britain for the current unrest, endangering embassy staff in the process.

"We believe the Iranian decision to expel two members of staff from the British embassy is baseless," the spokeswoman said.

"We think the Government of Iran is seeking to blame the UK and other outsiders for what is an Iranian reaction to an Iranian issue.

"This has a potential impact on our staff safety and is unacceptable. We have taken the decision to reciprocate."

Mr Brown's spokesman said of the UK diplomats being ordered to leave Tehran: "We view this decision as unjustifiable and baseless and decided to reciprocate."

He declined to disclose the identities or the posts of the British officials ordered back to London.

Earlier, the premier had defended his decision to speak out in the wake of the violence meted out to demonstrators in the Iranian capital following the country's disputed presidential election.

He told BBC Radio 4's the World at One: "We want a very good relationship with the Iranians, we also respect the fact that it's for the Iranian people themselves to choose who their government is.

"But when there is a sign of repression or where there is violence that's affecting ordinary people in the streets we have a duty to speak out and to say we want Iran to be part of the world, we don't want Iran to be isolated from the world.

"But that requires Iran to say it's taking seriously these issues we are raising with them and other countries are raising with them."

Iran's top electoral body today rejected calls to scrap the country's disputed election, ending hopes of a rerun demanded by angry opposition supporters.

The announcement by the Guardian Council on Iran's state-run English language Press TV was another sign the regime is determined to crush unrest, the worst since the 1979 Islamic Revolution, rather than seek compromise.

Government warnings to the protesters have intensified in recent days, with Iran's supreme leader ordering them off the streets and the feared Revolutionary Guards threatening a tough crackdown. At least 17 people have been killed in near-daily demonstrations.

President Barack Obama today "strongly condemned" the "unjust actions" by Iranian security forces, but reiterated his intention not to be seen to be meddling.

During a press conference at the White House, the president said: "The United States and the international community have been appalled and outraged by the threats, beating and imprisonments of the last few days. I strongly condemn these unjust actions and I join with the American people in mourning each and every innocent life that is lost."

But he added: "I have made it clear that the United States respects the sovereignty of the Islamic Republic of Iran and is not at all interfering in Iran's affairs."

The president has faced pressure from political opponents to toughen up his response to events in Iran.

Pressed by journalists during today's media briefing, Mr Obama declined to outline what consequences Iran could face if the bloody suppression of demonstrators does not end.

"We do not know yet how this is going to play out," he said.