Gordon Brown is threatening Iran with "tougher sanctions" unless it changes the direction of its nuclear programme.
In a speech today, the Prime Minister said the country had the right to pursue "civil nuclear power" but that its current programme was "unacceptable".
Echoing his address to the US Congress he said: "Iran has concealed nuclear activities, refused to co-operate with the IAEA, and flouted UN Security Council resolutions, reported Skynews.
"Its refusal to play by the rules leads us to view its nuclear programme as a critical proliferation threat.
"Iran therefore faces a clear choice: continue in this way and face further and tougher sanctions, or change to a UN overseen civil nuclear energy programme that will bring the greatest benefits to its citizens."
Mr Brown has announced his desire to create an international system to help all non-nuclear countries acquire nuclear power.
"Whether we like it or not," he said, "we will not meet the challenges of climate change without the far wider use of civil nuclear power."
Iran recently succeeded in enriching a tonne of uranium - enough for at least one nuclear warhead.
But Tehran has always insisted it is not planning to build a bomb and only wants nuclear energy to generate power.