Foreign Secretary William Hague has said Iran has been testing missiles capable of carrying a nuclear warhead.
He told Parliament the covert ballistic missile tests and rocket launches contravened a UN resolution.
The Iranians openly test-fired 14 surface-to-surface missiles with a range of 2,000km (1,250 miles) on Tuesday as part of a 10-day programme of military exercises.
Its Foreign Ministry said none of these missiles had had nuclear capability.
But Britain believes Iran has also conducted at least three secret tests of medium-range ballistic missiles since October.
The United States, Britain and other Western nations have long believed Iran's nuclear programme is aimed at developing atomic weapons - but this is rejected by Tehran.
Mr Hague promised the UK would increase the pressure on Iran to negotiate over its nuclear programme.
"Iran has also been carrying out covert ballistic missile tests on rocket launchers, including testing of missiles capable of delivering a nuclear payload in contravention of UN resolution 1929," he said.
"And it has announced that it intends to triple its capacity to produce 20% enriched uranium.
"These are enrichment levels far greater than is needed for peaceful nuclear energy.
"We will maintain and continue to increase pressure on Iran to negotiate an agreement on their nuclear programme."