Tehran will today be offered the chance of international assistance to develop a peaceful nuclear energy programme if it halts uranium enrichment — or face punishing sanctions within a month.

Javier Solana, the European Union chief, will deliver the message to Iranian leaders today as the head of a six-member international delegation of senior diplomats from Russia, China, Britain, France and Germany. The “carrot and stick package” outlined by British officials this week offers Iran a range of incentives to begin negotiations but not until it has halted all Iranian enrichment.

Officials acknowledge that the package differs little to that offered in 2006 but hope that its timing, greater international support for punitive measures should Iran refuse and more specific proposals of help may induce the Iranians to consider it more seriously. The package will be accompanied by a strongly worded letter signed by the foreign ministers of all the so-called “EU3 plus 3” countries, including the United States, the only one not sending a representative to Tehran. The Bush Administration refuses to negotiate directly with Tehran while it continues to defy the world over its nuclear programme.

Mr Solana will meet Manouchehr Mottaki, the Foreign Minister, and Saeed Jalili, Iran’s top nuclear negotiator — but crucially not President Ahmadinejad, who is seen as an obstacle to a settlement. The team hope to exploit divisions within Iran’s hardline leadership to sell the plan, pitching it towards more moderate leaders among whom a debate about the wisdom of an offensive nuclear programme has emerged.

Unlike 2006, the team plan to make the details of the package public at a press conference in Tehran to make sure the debate can continue outside the immediate circle of their meetings. The contents of the 2006 package were never released and remained a secret to many even within the upper echelons of government. “This way they can not misrepresent it,” an official said.

Officials have admitted that they are not confident of Iran’s compliance but that the offer of incentives had been necessary to secure the agreement of China and Russia to punitive measures should they reject the plan.

The trip comes as President Bush, in a tour of Europe, again hinted at the threat of military action, cautioning that Iranian belligerence must be firmly opposed and Iran must no be allowed to have a nuclear weapon.

This weekend’s trip, however, focuses on the threat of economic punishment rather than military action. New EU sanctions could be in place within a month if Iran rejects the package outright. Should it simply ignore the proposals, the sanctions could be introduced in little more than six weeks.

That would pave the way for a vote at the UN Security Council to impose its own restrictions, which would focus on making it harder for Iranian companies to do business around the world.

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