Iran has been testing missiles with nuclear capabilities that could knock out U.S. bases in the Gulf.
Britain accused Iran on Wednesday of conducting at least three secret tests of ballistic missiles capable of delivering nuclear payloads in breach of UN resolutions, drawing a quick denial from Tehran.
Foreign Secretary William Hague told the House of Commons the tests contravened resolutions forbidding Iran from developing a nuclear weapons program, according to The Telegraph newspaper.
Iran has "been carrying out covert ballistic missile tests and rocket launches, including testing missiles capable of delivering a nuclear payload," Hague said.
While Hague did not specify where and when the tests had taken place, British officials said at least three secret tests had been detected since October. London had already informed the United Nations but only made the information public on Wednesday, they said.
Iran dismissed the allegations. A spokesman for the Iranian foreign ministry said: "None of the missiles tested by Iran is capable of carrying a nuclear warhead."
But the British Foreign Office accused Tehran of "provocative acts" and breaching its obligations under UN Security Council resolutions which prohibit it from developing ballistic missiles to deliver nuclear warheads.
Iran is currently showing off its medium-range ballistic missile technology with 10 days of publicized tests, but British officials said the secret launches were not part of the visible program.
One of the missiles publicly tested this week is capable of reaching Israel and the Gulf states, The Telegraph reported.
The West and some of Tehran's Arab neighbors accuse Iran of conducting a clandestine program to develop nuclear arms, but the Shiite regime says its activities including uranium enrichment are for peaceful energy and its missile tests are purely defensive.
A UN experts' report leaked to the media last month said Iran conducted secret ballistic missile tests in October and February, according to The Associated Press.
Iran launched a liquid-fueled Shahab 3 missile, with a range of 560 miles (900 kilometers), and one or two solid-fueled Sejil 2 missiles, with a range of 1,240 miles (2,000 kilometers), it said.
The experts said they believed both missiles were capable of carrying a nuclear warhead but did not say if the tests were successful.