Gordon Brown has condemned Iran's latest missile test and warned it faced tougher sanctions.
The Prime Minister discussed the development when he met UN Secretary General Ban Ki-moon at climate change talks in Copenhagen.
He said afterwards: "I have expressed to him and he has also expressed concern about the test of a long-range missile by Iran.
"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."
Iran claimed that it had successfully tested its Sajjil-2 missile, a high-speed, surface-to-surface missile with a range of about 1,200 miles.
That range places Israel, Iran's sworn enemy, well within reach and increases the threat against south-eastern Europe.
Iran has intensified its missile development program in recent years to the anger of Israel and the West. Tehran is also accused of trying to build a nuclear weapon.
Iran, which is under several sets of UN sanctions over its nuclear program, denies the charges and says its nuclear program is aimed solely at generating electricity.
