President Bush is sending a top US diplomat to meet Iran’s nuclear negotiator this weekend, a major break with his hardline stance towards Tehran and the closest contact between the countries since the 1979 Iranian Revolution.

In a significant departure from Mr Bush’s long-standing refusal to talk to Tehran until it has abandoned its nuclear enrichment programme, William Burns, the Under-Secretary of State and America’s third most senior diplomat, will travel to Switzerland to attend talks between Iranian and European officials on Saturday.

The move comes as the US considers establishing a diplomatic presence in Tehran for the first time since the closure of its Embassy in 1980. As The Times reported last week, US officials had discussed sending consular staff to the Swiss Embassy in Tehran, to handle visa applications.

Mahmoud Ahmadinejad, the Iranian President, has welcomed the plan. “We have not received any official request for the moment, but we think that the development of relations between the two peoples is something correct,” Mr Ahmadinejad said on Sunday. “Any proposal in this direction can be examined and we will receive it favourably.”

The diplomatic overtures, which come amid signs that Iran is preparing to suspend its uranium enrichment activities, are the latest evidence of a more multilateral approach to foreign policy that has marked much of Mr Bush’s second term. It follows a recent deal with North Korea over its nuclear programme.

Until yesterday the US had refused even to participate in preliminary discussions with Iranian officials. The White House softened its stance because it believes that a deal in which Tehran will freeze its nuclear programme is within reach before Mr Bush leaves office.

The issue of how to thwart Tehran’s nuclear ambitions has already become a point of contention in the race to succeed Mr Bush. Barack Obama, the Democratic candidate, has said that he would be open to direct talks with Iran if it met certain conditions. His Republican opponent, John McCain, opposes direct talks.

Mr Burns will attend the Geneva meeting between Javier Solana, the EU’s top foreign policy official, and Saeed Jalili, Iran’s chief nuclear negotiator. Mr Solana is seeking a definitive response from Tehran to the latest package of incentives for suspension offered last month by the five permanent members of the UN Security Council plus Germany.

Under the “freeze for freeze” offer, Iran would not enrich any uranium for six weeks, while the US, Britain, France, Germany, Russia and China would not seek additional economic sanctions. During this period, both sides would thrash out the terms for preliminary negotiations over a permanent halt.

US officials insisted that Mr Burns would not hold separate talks with Iranian officials, and that he would reiterate Washington’s insistence that Iran permanently halt its enrichment programme. “This is a one-time deal,” a State Department official said.

The reaction of Washington’s hardliners demonstrated what a significant break with past policy Mr Burns’s attendance represents. “This is a complete U-turn and very disappointing to say the least,” John Bolton, a neoconservative and Mr Bush’s former Ambassador to the UN, told The Times.

“Under the freeze-for-freeze deal Iran only has to not increase its nuclear material. This is an acknowledgment and validation of its existing enrichment activities.”

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