Iran has conducted covert tests of ballistic missiles in addition to a 10-day program of public military manoeuvres, British officials alleged on Wednesday.
Foreign Secretary William Hague told the House of Commons that there had been secret missile experiments, but did not specify precisely when the tests had taken place.
Iran has "been carrying out covert ballistic missiles tests and rocket launches, including testing missiles capable of delivering a nuclear payload," Hague said.
Britain believes Tehran has conducted at least three secret tests of medium-range ballistic missiles since October as part of an apparent escalation of its nuclear program and scrutiny from the International Atomic Energy Agency.
Iran is currently displaying its military hardware in a series of war games in an apparent show of openness, and on Tuesday fired 14 missiles in public tests.
However, the UK believes that the covert missile tests show Iran's leaders are seeking to avoid scrutiny over the real extent of their weapons programs.
"On the back of the recent IAEA report and the unanswered questions about its nuclear program, they only serve to undermine further Iran's claims that its nuclear program is entirely for civilian use," said a Foreign Office spokesman, on a customary condition of anonymity in line with policy.
An IAEA report last month listed "high-voltage firing and instrumentation for explosives testing over long distances and possibly underground" as one of seven "areas of concern" that Iran may be conducting clandestine nuclear weapons work.
Hague also said Britain was concerned over Tehran's decision to increase its capacity to enrich uranium to a higher level at the Fordo site near the holy city of Qom in central Iran.
"It has announced that it intends to triple its capacity to produce 20 per cent enriched uranium. These are enrichment levels far greater than is needed for peaceful nuclear energy," Hague said.
Low-enriched uranium – at around 3.5 per cent – can be used to fuel a reactor to generate electricity, while uranium enriched to around 90 per cent purity can be used to develop a nuclear warhead.
Enriching to 90 per cent can be done much more easily from material enriched to 20 per cent purity than from low-enriched material.
Some Western officials claim Iran is accelerating its attempts to produce a nuclear weapon, partly because leaders have been shaken by protests against authoritarian regimes across the Middle East.
Iran and the West remain in dispute over its nuclear program. The US and its allies insist it is aimed at developing atomic weapons – a charge Iran rejects.
"We will maintain and continue to increase pressure on Iran to negotiate an agreement on their nuclear program," Hague said.
Hague also condemned Tehran over its support of the violent crackdown on anti-regime demonstrations in Syria.
"Iran continues to connive in the suppression of legitimate protest in Syria and to suppress protests at home," he said.