Gordon Brown last night proposed a worldwide ban on companies developing Iran’s oil and gas fields if it failed to curb its nuclear ambitions.

He promised to take the lead in seeking tougher penalties through the United Nations and the European Union as Britain and the United States seek to increase the pressure on Tehran.

In his first major speech on foreign policy the Prime Minister said that Iran had a choice — confrontation with the international community and stringent sanctions against it; or dropping its nuclear plans, ending support for terrorism and having a transformed relationship with the world.

Unless imminent reports from the EU and the International Atomic Energy Agency suggested movement from Iran, there would be stronger sanctions, including on oil and gas investment and the financial sector. “Iran should be in no doubt about our seriousness of purpose,” he said.

Although Mr Brown does not rule out military action, and spoke last night about the occasional need for “hard-headed intervention”, he clearly believes that the diplomatic route should be followed. Wearing white tie for the first time at a City event, he set out what he called Britain’s “agenda for a hard-headed internationalism”.

Speaking at the Lord Mayor’s Banquet at Guildhall, Mr Brown also:

— Told America that it was Britain’s most important bilateral relation. “I have no truck with anti-Americanism in Britain or elsewhere,” he said;

— Urged President Musharraf of Pakistan to restore the Constitution, release all political prisoners and step down as Chief of Army Staff;

— Proposed reform of the big international institutions, including the UN, the G8, the International Monetary Fund and the World Bank;

— Called for an international standby civilian force, including police and judges, to help to rebuild failed states, with reconstruction and development given equal weight with peacekeeping in international bodies.

Mr Brown said: “My approach is hard-headed internationalism: internationalist because global challenges need global solutions and nations must co-operate across borders, often with hard-headed intervention, to give expression to our shared interests and shared values.”In an apparent reassurance to the US after ministerial criticism of President Bush, he said that Britain’s partnerships were not in competition with each other but “mutually reinforcing”. He said: “It is no secret that I am a life-long admirer of America . . . and it is good for Britain, Europe and the wider world that today France and Germany and the EU are building stronger relationships with America.”

In a further reference to Iran, Mr Brown proposed that the active providers as well as the potential users of nuclear materials should be held to account. He suggested agreed access to a nuclear fuel bank to help non-nuclear states to acquire the new sources of energy they needed. But the offer would be made only if those countries renounced nuclear weapons.

He rejected isolationism and said that it was possible to contemplate a global society that empowered people. “We cannot any longer escape the consequences of our interdependence. The old distinction between ‘over there’ and ‘over here’ does not make sense of this interdependent world.”

Turning to Pakistan, Mr Brown said: “We call on President Musharraf of Pakistan to restore the Constitution and implement the necessary conditions to guarantee free and fair elections on schedule in January.”

Don McKinnon, the Commonwealth Secretary-General, last night threatened to suspend Pakistan from the 53-member group, saying that it had “seriously violated the Commonwealth’s fundamental values”. He called for the resignation of Mr Musharraf as army chief, the release of political detainees, the removal of press restrictions, a move towards elections and restoration of the Constitution.

The Pakistan High Commission in London said that “decisions regarding the transition will be taken in accordance with Pakistan’s national interests and requirements, not deadlines imposed from outside”.

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