As the Security Council unanimously adopted a historic resolution calling for nuclear disarmament, the Prime Minister told the summit in New York that the international community could not “stand by” and let the non-proliferation regime be broken,

Mr Brown’s tough rhetoric on having to “draw a line in the sand” on the UN Nuclear Non-Proliferation Treaty, and his formal offer to cut the UK Trident submarine fleet, was overshadowed by reports of a “snub” by US President Obama to the embattled British Prime Minister.

Both the White House and Downing Street yesterday rushed to deny any rift in the “special relationship” after reports that Mr Brown’s Downing Street team had sought bilateral talks with President Obama at the UN on five occasions and had been rebuffed.

The two men ended up having a 15-minute discussion in the kitchens of the UN after a dinner, but there was no handshake for the cameras that Mr Brown had sought.

The snub was immediately linked to the decision by the Scottish Government to free the Lockerbie bomber, Abdelbaset Ali Mohmed Al Megrahi.

Yesterday both leaders were careful to be filmed smiling, greeting and chatting with each other at the UN Security Council meeting. Mr Obama made a point of “walking and talking” with Mr Brown and both their offices insisted that the working relationship between them was still good.

Mr Brown, clearly angered to be distracted from his agenda on global warming, nuclear weapons and the economy on to personality politics, took to the airwaves to deny that anything was amiss.

“President Obama and I have the strongest working relationship and the strongest friendship. I am not only very confident about the strength of the relationship between our countries and I am very confident about the relationship between the two of us.”

Mr Brown avoided any further embarrassment when Libya’s Colonel Gaddafi failed to appear for the meeting as the African Union representative on the Security Council. His place was instead taken by Libya’s UN ambassador.

If proof of the special relationship and its nature were needed, it was demonstrated in Britain leading the charge at the Security Council against Iran – effectively doing America’s bidding – while the US President took the commanding heights without issuing specific threats.

President Obama said the resolution on non-proliferation was “not about singling out an individual nation”. He added: “It is about standing up for the rights of all nations who do live up to their responsibilities.”

Mr Obama and French President Nicolas Sarkozy identified North Korea, which has tested nuclear weapons,

and Iran, suspected of a secret weapons programme, as obstacles to a safer world.

But Mr Brown went further. He said: “We cannot stand by when Iran and North Korea reject the opportunities of peaceful, civil nuclear power and instead take steps to develop nuclear weapons in a way that threatens regional peace and security. As evidence

of [Iran’s] breach of international agreements grows, we must now consider far tougher sanctions together.”

Russia has now agreed that further action has to be taken against Iran, but China, which also has a security council veto and is a major importer of Iranian oil, warned further pressure may be counter-productive. Iran issued a statement accusing America of conspiring against it but remained open to international negotiation.

Following the Security Council meeting, a No 10 spokesman said decisions on the “next steps” would be taken after talks next week in Istanbul between Iran and the E3+3 group of Britain, France and Germany, plus the US, Russia and China.

“The meeting on October 1 that will take place between the E3+3 and Iran is a vital moment for the international community’s concerns about its nuclear programme,” the spokesman said.

Mr Brown and Mr Obama went on to co-chair a meeting of the Friends of Democratic Pakistan and will be together at the G20 summit in Pittsburgh today.