"It hit the defined target," state television reported, without giving further details.
The two-stage Sejil, powered by solid fuel, is capable according to Iran of travelling 2,000 kilometres (1,240 miles), which would put arch-foe Israel, most Arab states and parts of Europe, including much of Turkey, within range.
British Prime Minister Gordon Brown said the test of the missile was a matter of "serious concern" and makes the case for "moving further on sanctions."
Brown is in Copenhagen for the United Nations climate summit, where he discussed the development with UN Secretary General Ban Ki-moon.
"I have expressed to him and he has also expressed concern about the test of a long-range missile by Iran," Brown said, according to his Downing Street office in London.
"This is a matter of serious concern to the international community and it does make the case for us moving further on sanctions.
"We will treat this with the seriousness it deserves."
Iranian Defence Minister Ahmad Vahidi said Wednesday's test was aimed at bolstering the Islamic republic's defences and insisted missiles would not be used for offensive purposes.
"The missile test that we witness today is one ring of our defence capability chain in line with increasing the armed forces' deterrent power," Vahidi was quoted as saying by the Fars news agency.
Iran's missile capability, he added, is "merely defensive and for peace, stability and calm in the region and will not be used against any country."
Vahidi said the latest version of the Sejil was "impossible to destroy" due to the "very high speed" at which it hits the ground. It also has anti-radar capability.
The United States and its regional ally Israel have not ruled out a military option to stop Tehran's controversial nuclear drive.
Iran has in the past threatened to target US bases in the region and to block the strategic Gulf Strait of Hormuz waterway for oil tankers if its nuclear sites are attacked.
Iran is under three sets of UN sanctions for its refusal to suspend enrichment and risks a further round after rejecting a UN-brokered deal to send its low enriched uranium abroad to be further refined into fuel for a research reactor.
Enrichment lies at the heart of fears over Iran's controversial atomic work as the process to make nuclear fuel can also be used to make the fissile core of an atom bomb in much higher purifications.
Further pressuring the Islamic republic, the US House of Representatives on Tuesday overwhelmingly approved sanctions legislation aimed at forcing Iran to freeze the programme by depriving the Islamic republic of petrol.
The measure would empower US President Barack Obama to effectively block firms that supply Iran with refined petroleum products, or the ability to import or produce them at home, from doing business in the United States.
In late September the Islamic republic test-fired the previous version of Sejil 2 (Lethal Stone) and along with another similar-range weapon, the Shahab-3, and a host of shorter-range missiles as part of a military exercise.
The tests which came ahead of key talks between Iran and six world powers in Geneva drew the ire of the West, with Washington describing them as "provocative."
