IRAN will come under growing pressure this week to accept a Russian-brokered compromise to end the stand-off over its nuclear ambitions after the disclosure that it obtained the know-how to make the core of a nuclear warhead 18 years ago.

Under a proposal backed by America and Britain, Iran would be allowed to continue to refine uranium at its own nuclear facility, provided it was subsequently enriched in Russia. The material would then be sent back to Iran to fuel a Russian-built nuclear reactor there.

The plan is expected to be discussed at a meeting of the International Atomic Energy Agency (IAEA) this Thursday.

Enriching uranium under Russian scrutiny would ensure that the process went far enough for civilian purposes but not for use in nuclear weapons, officials believe. This would go some way towards assuaging fears in the West that Iran’s civilian nuclear industry is little more than cover for a bomb-making programme.

In what is seen as a important reversal of policy, America — which believes Iran wants to make a bomb — appears to have realised that its hardline stance towards Tehran is failing to resolve a two-year crisis.

American enthusiasm for the proposal was evident when President George W Bush met Vladimir Putin, Russia’s president, in South Korea last week.

“We hope that over time, Iran will see the virtue of this approach and it may provide a way out,” said Stephen Hadley, Bush’s national security adviser, after the meeting.

The plan was also believed to have been discussed when senior British, American, French, German, Russian and Chinese officials met in London on Friday.

Iran, which has rejected any controls on its nuclear activity, initially rejected the plan, but has since indicated that it may consider it.

Western diplomats say reading Tehran’s intentions has been increasingly difficult following the election in June of Mahmoud Ahmadinejad, a hardline populist, as president. Ahmadinejad has thrown the country’s leadership into chaos in recent weeks by instituting a purge of top officials and diplomats.

Western diplomats feared that taking too tough a line against Iran could risk unifying the country behind the president, but the latest revelations about its knowledge of nuclear weapon-making has added to concern. As a further act of defiance, Iran also announced last week that it was processing another batch of uranium for its nuclear programme.

In a report leaked ahead of Thursday’s meeting, the IAEA said Iran admitted receiving a document in 1987 showing how to cast “enriched, natural and depleted uranium metal into hemispherical forms” — interpreted by experts as meaning the design for the core of a nuclear warhead. It would not be sufficient to build an entire weapon, however.

Iran claimed it had not actively sought the information. Officials said it had been provided “on the initiative of a procurement network”, believed to have been run by AQ Khan, the father of Pakistan’s nuclear programme.

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