A US National Intelligence Estimate released last December said Tehran had frozen its atomic programme in 2003.
But documents presented to the International Atomic Energy Agency (IAEA) suggest the work continued.
Ali Asghar Soltanieh, Iran's ambassador to the IAEA, angrily dismissed the documentats as "forgeries".
Simon Smith, Britain's ambassador to the IAEA, said material presented to the IAEA in Vienna came from multiple sources and included designs for a nuclear warhead, plus information on how it would perform and how it would fit onto a missile.
"Certainly some of the dates that we were talking about... went beyond 2003," he said.
Mullahs with nukes. What to do?