The Government remains "gravely concerned" over Iran's nuclear ambitions, the Foreign Office has said.
Junior Foreign Office minister Alistair Burt said President Mahmoud Ahmadinejad's hardline Islamist regime had taken "a significant step towards weapons grade enrichment".
He vowed to press Iran into negotiations over its uranium enrichment programme.
Mr Burt said: "We continue to be gravely concerned about the Iranian nuclear programme."
He added: "It will now be essential to increase the pressure on Iran to convince it to negotiate seriously."
Mr Burt's comments came in written Parliamentary answers to Tory MP David Amess.
The minister said the most up-to-date International Atomic Energy Agency report found Iran had produced 3,183kg of 3.5% low-grade uranium and 33kg of 20% enriched uranium.
Mr Burt said Iran "was still failing to address the wide range of concerns over possible military dimensions of its programme".
He said the UK, China, France, Germany, Russia and the US - known as the P5+1 group - were "committed to pursuing a diplomatic solution and that the door to further dialogue remained open".