BRITAIN is pressing for a United Nations resolution that would open the way for punitive sanctions and even the use of force if Iran were to refuse to halt its controversial nuclear programme.

In a confidential letter obtained by The Times, a leading British diplomat outlines a strategy for winning Russian and Chinese support by early summer for a so-called Chapter VII resolution demanding that Iran cease its nuclear activities.

If the Government in Tehran refused to comply with such a resolution, the UN Security Council would be legally compelled to enforce it.

The strategy marks a significant hardening of the Government’s position. It contrasts with public statements by Jack Straw, the Foreign Secretary, this month. On March 13 he insisted that military action was “inconceivable” and that the dispute with Iran “has to be resolved by peaceful democratic means”.

The confidential letter was written only three days later by John Sawers, the political director at the Foreign and Commonwealth Office, and sent to his American, French and German counterparts.

“They (the Iranians) will need to know that more serious measures are likely,” wrote Mr Sawers, in a letter first leaked to the Associated Press. “This means putting the Iran dossier on to a Chapter VII basis.”

He suggested making a suspension of all uranium enrichment by Iran “a mandatory requirement of the Security Council, in a resolution we would aim to adopt, I say, early May”.

Mark Fitzpatrick, an expert on nuclear proliferation at the International Institute for Strategic Studies, said that securing a Chapter VII resolution would provide the international community with a “stick” it could use against Iran. “It would be an important breakthrough,” he said. “It would open the door to sanctions and other measures.”

Before wielding any stick, however, Mr Sawers proposed that the international community give Iran a final chance in the form of a “revised offer” of incentives as a face-saving solution to allow it to back down peacefully.

The two-track diplomacy was devised by the British in an attempt to reach a compromise between the five permanent members of the UN Security Council: America, China, Britain, France and Russia.

The US favours moving straight to a tough resolution that would punish Iran if it failed to halt its nuclear programme. Russia and China, which both have important commercial ties with Iran, favour a slower, less confrontational approach handled by the International Atomic Energy Agency, the nuclear watchdog.

“We are not going to bring the Russians and Chinese to accept significant sanctions over the coming months, certainly not without further efforts to bring the Iranians around,” Mr Sawers wrote.

“In parallel with agreeing a new proposal, we will also want to bind Russia and China into agreeing to further measures that will be taken by the Security Council should the Iranians fail to engage positively,” he wrote.

But the British initiative has so far failed to bring the parties together. On Monday Mr Sawers hosted talks at the UN between the five permanent members and Germany which broke up without agreement.

The US refused to take steps that would reward Iran or ease pressure on the regime. Russia, which has billions of pounds in contracts to supply Iran with civilian nuclear technology and sophisticated arms, and China, which has multibillion-pound deals to import Iranian oil and gas, rejected any move that could lead to punitive action. Yesterday follow-up talks at the UN were postponed.

Mr Sawers anticipated the hurdles in his letter. “I suspect we will need a meeting at ministerial level anyway to get agreement to this sort of approach, including an early Chapter VII resolution,” he wrote.

Nevertheless the international community will have to reach agreement if it hopes to halt Iran’s uranium enrichment work, which it resumed in February at Natanz.

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