By Liz Stephens and Alex Stevenson
Gordon Brown says the international community is drawing a "line in the sand" against Iran on nuclear issues, after accusing it of developing a secret atomic facility for military use.
The prime minister made the claims about the covert underground facility for making nuclear fuel in a joint statement with US president Barack Obama and French president Nicholas Sarkozy.
The joint revelation, made in Pittsburgh as Mr Brown chaired the G20 summit, claimed Iran has hidden the nuclear plant from inspectors for years.
Current intelligence known to senior G20 nations, including Russia, suggested the facility could not have been in use for civil purposes because of insufficient levels of production.
Although it is not believed to be yet in operation, there is speculation that it could be running next year. The three leaders demanded Iran opens up the facility for immediate inspection by October 1st, when the five permanent UN security council members will meet with Iran in Geneva.
"Iran must abandon any military ambitions for its nuclear programme," Gordon Brown said.
Accusing the Middle Eastern state of "serial deception over several years", he said the levels of "deception by Iran and betrayal" would "shock and anger the whole international community".
It is understood that Iran wrote to the International Atomic Energy Agency on Monday and made reference to a "pilot plant".
However, Iran's president Mahmoud Ahmadinejad did not mention the plant at the UN and asserted that Iran had cooperated fully with inspectors and that allegations of a nuclear weapons program were false.
"Iran's nuclear programme is the most urgent nuclear proliferation challenge we face today," Mr Brown added.
"On October 1st Iran must engage with the international community and join as a partner - if it does not, further sanctions will be made."
Any sanctions are expected to target Iran's financial capabilities and their ability to receive any technological equipment which can be used for military purposes.