Mr Solana arrived in Tehran last night last night as the messenger to convey a deal agreed in talks among the five permanent members of the UN Security Council, plus Germany, in Vienna last Friday. The former Nato chief told reporters at Tehran airport that the West wanted "a new relationship" with Iran and that the package would "allow us to engage in negotiations based on trust, respect and confidence".

Details of the proposals, drawn up by Britain, France and Germany and backed by the United States, Russia and China, have been kept secret, but an early draft suggests that if Iran agrees to stop enriching uranium, the major powers would offer it help in building nuclear reactors and guarantee a supply of nuclear fuel.

The offer contains the implicit threat of UN Security Council sanctions if Iran continues to insist on enriching uranium - a process that can produce fuel for generating electricity or material for making nuclear bombs.

But with both China and Russia keen to avoid any more explicit threats against Tehran, the major powers have focused on incentives to draw Iran back into the fold. The package is thought to include the offer of European Airbus aircraft. The United States, which offered last week to enter direct talks with Iran for the first time since the Islamic revolution of 1979, has reportedly sweetened the offer by saying that it would lift some bilateral sanctions, including a ban on the sale of Boeing passenger aircraft and spare parts.

Iran has so far insisted that as a signatory to the Nuclear Non-Proliferation Treaty it is entitled to enrich uranium for peaceful purposes. But the world is suspicious of its nuclear plans because it concealed significant aspects of its programme for many years.

Mr Solana, the EU's foreign policy supremo, was due to meet his counterpart Manouchehr Mottaki, the Iranian Foreign Minister, later in the day before leaving Iran, but officials said he was not expected to go beyond the simple exposition of the package. "This trip is not a negotiating trip. Mr Solana is here just to present the package to Iranian officials," said an official at the Supreme National Security Council.

Iranian officials have sent conflicting signals on the initiative, reflecting a possible struggle within the leadership on how to react. While Iran¡¯s supreme leader, Ayatollah Ali Khamenei, sent oil prices higher by threatening to disrupt world supplies if the United States attacked Iran, President Mahmoud Ahmadinejad said the new initiative was welcome and promised that his government would consider the package seriously.

Iran announced in April that it had enriched uranium for the first time, using 164 centrifuges, after breaking a UN moratorium on enrichment. The country would need tens of thousands of centrifuges to produce adequate fuel for a nuclear reactor or material for a warhead.

Iran has said that it intends to move toward large-scale enrichment involving 3,000 centrifuges by late 2006, a move that American officials say could give it a nuclear weapon within ten years.

It is not clear how long Iran will take to respond to the major power plan. Tony Snow, the White House spokesman, said yesterday: "I would counsel patience. At this point, as we've said all along, let's give it time. Let's let the Iranians take a look at what the offers are, at the incentives and disincentives."