Gordon Brown lurched from being hailed as a global statesman to intense embarrassment tonight, after it emerged US President Barack Obama had turned down no fewer than five requests from Downing Street to hold a bilateral meeting at the United Nations in New York or at the G20 summit starting in Pittsburgh today.

The prime minister, eager to portray himself as a leading player on the international stage in America this week, was also forced to play down suggestions from inside his own party that he might step down early, either due to ill health or deteriorating eyesight.

There have been tensions between the White House and No 10 for weeks over Brown's handling of the Scottish government's decision to release the man convicted of the Lockerbie bombing, Abdelbaset al-Megrahi.

Brown's efforts to secure a prestigious primetime slot for his keynote speech at the general assembly in New York were also thwarted when the Libyan leader, Colonel Gaddafi, delivered a 100-minute speech to the UN, massively running over Brown's 15 minute slot.

Brown had not only been seeking a bilateral meeting with Obama, but feelers were also sent out to hold a joint press conference, an event that would have boosted Brown's efforts to offer himself as a linchpin of international diplomacy. Government sources said that Britain even changed its policy on swine flu immunisation in Africa to match that of the Obama administration last week, in an attempt to rebuild relations.

No 10 denied there had been any hint of a snub, saying Obama and Brown had plenty of chances to talk as they sat next to one another at the summits. They insisted they were working closely on issues such as future economic regulation, bankers' bonuses, nuclear non-proliferation and climate change. Brown himself insisted: "I do say that the special relationship is strong, it continues to strengthen."

But Obama has held bilateral meetings in New York with the Chinese president, Hu Jintao, the Russian president, Dmitry Medvedev, and the new Japanese prime minister, Yukio Hatoyama.

News of the five spurned approaches compounded a miserable day for Brown at home which saw a parliamentary aide resign over the prime minister's refusal to sack Lady Scotland, the attorney general, after she was fined £5,000 for employing an illegal immigrant, as well as a withering attack by the former home secretary Charles Clarke.

Stephen Hesford, Labour MP for Wirral West, told Brown in a resignation letter: "In my view, the facts of the case do not matter. It is the principle which counts, particularly at a time when the public's trust of Whitehall is uncertain to say the least. We have to be seen to be accountable."

Brown was also savaged by Charles Clarke, who told the Evening Standard that in his view Brown's leadership risked letting "the whole Labour ship crash on to the rocks of May 2010 [the expected date of the general election] and sink for a very long time". He said he hoped rumours that Brown would quit would come true. "I think his own dignity ought to look to that kind of solution."

In two interviews , Brown was forced for the first time to field questions about his health. "My sight is not at all deteriorating," he told NBC.

Asked on BBC Radio 5 Live whether he might quit for health reasons, the prime minister replied: "I am healthy and I am very fit. I run a lot to keep fit and I will continue to keep fit.

"I keep going. I have got a job to do. I have got work to do. We have got to meet this challenge."

In his own speech to the UN, Obama promised an end to the unilateralism marked by the previous Bush administration, an approach that saw US and Britain working in tandem. In remarks that suggest Obama will focus on broadening American alliances across the globe, he said: "The time has come for the world to move in a new direction … a new era of engagement based on mutual interest and mutual respect."

Last week, the White House had unusually briefed that Obama had told Brown in a phone conversation that he disapproved of the release of the Lockerbie bomber on compassionate grounds, something No 10 had not highlighted. Megrahi was released on the basis that he had three months to live, and then received a hero's welcome as he returned to Tripoli.

Brown had said he respected the release, but insisted undertakings by the Libyans that the return would be low-key had been broken.

In an attempt to distance himself from the Libyans, Brown moved to toughen his position against Colonel Gaddafi after Libya's leader theatrically tore up the UN charter in his address. Gaddafi said the security council should be renamed "the terror council".

Brown countered in his speech in New York later: "I am here to reaffirm the UN charter, not to tear it up. I call on everyone to support its universal principles."

He urged world leaders to recognise that the next six months presented tests on climate change and terrorism that were as huge as the banking crisis. On climate change he said: "If we miss this opportunity to protect our planet, we cannot hope for a second chance some time in the future. There will be no retrospective global agreement to undo the damage we have caused. This is the moment now to limit and reverse climate change we are inflicting on future generations."

He added: "If the poorest and most vulnerable are going to be able to adapt, if the emerging economies are going to embark on low-carbon development paths, if the forest nations are going to slow and stop deforestation, then the richer countries must contribute financially."

No 10 again denied that the prime minister had been snubbed last night. A spokesman said the stories were "without foundation. As we have said throughout the week, the prime minister and President Obama are having a number of meetings throughout the week. These included a wide-ranging discussion following last night's climate change dinner. They will also be co-chairing an important meeting on Thursday on Pakistan and the fight against terrorism. As the prime minister has already said, there will be further meetings at the G20 summit in Pittsburgh."