The Iranian parliament on Sunday gave the government a two-week deadline to expel the British ambassador in Tehran, in retaliation for London's decision to cut all financial ties with Iranian banks.
Some 87 percent of lawmakers approved a bill demanding the expulsion of the ambassador and the downgrading of diplomatic ties.
In London, the Foreign Office described the decision as "regrettable" and said it would be responding "robustly" if the Tehran government acted on the lawmakers' vote.
"This unwarranted move will do nothing to help the regime address their growing isolation or international concerns about their nuclear program and human rights record," it said.
"If the Iranian government acts on this, we will respond robustly in consultation with our international partners."
The Iranian parliament has called more than once on the government to downgrade and even cut all ties with London.
The Foreign Ministry has so far ignored such calls. But if this bill is also approved by the senate-like Guardian Council, the government will be legally bound to implement it.
The parliament also recommended reducing economic and commercial ties to a minimum.
Last week, the parliament said that normal relations with London were no longer justified, after a British decision to halt all financial contacts over the banks' involvement in Iran's alleged nuclear weapons program.
The British move followed a report by the International Atomic Energy Agency two weeks ago that effectively accused Iran of having tested designs for a nuclear warhead.
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