The PM’s speech to the party faithful in Brighton has already gone through a number of trustees. But the comment last week by John Prescott, former deputy prime minister, is said to have struck an unexpected chord with Brown’s senior advisers.

Prescott’s aide-memoire for those around Brown was to make sure they knew how best to sell him: Brown has to be Labour’s “global giant” claimed Prescott. Anything less will mean Brown being undersold – with Brown and Labour losing out.

At the United Nations last week, Downing Street, working alongside Britain’s top UN diplomats, were forced to mount a concerted rescue operation to save Brown from humiliation. A begging bowl was put out five times, and rejected by Barack Obama’s UN delegation.

The request was simple enough: the British prime minister wanted a one-to-one meeting with the US president. Far from looking like Prescott’s “global giant”, Brown was left with descriptions that had he been “snubbed” by Obama. Only after a last-minute photo opportunity was arranged with the two leaders did Downing Street’s anger, firmly directed at the UK media, ease up.

Why the refusal? Former British ambassador to the US Christopher Meyer, who was alongside Tony Blair in the run up to the war with Iraq and who witnessed most of the deal-making Blair made with Bush, said “unsentimental” American directness was probably the root cause of the denied snub. “They know perfectly well that the British prime minister is on his last legs and that they will, within a matter of months, be dealing with a new leader, probably David Cameron,” said Meyer. The message was that Washington would not want to expend too much energy on an expiring foreign leader.

One diplomatic caveat hovers over Meyer’s analysis. Over the next few crucial months, Obama may need to look to Brown as his “global” ally and Brown will be more than happy to take the role as international global statesman as they both focus on the crisis of Iran’s nuclear ambitions.

By accident, and certainly not by any choreography, Brown has been handed a global crisis that could transform his last months in office. The crisis centres on a secret Iranian nuclear enrichment plant and demands that International Atomic Energy Agency (IAEA) inspectors be given access to the hidden plant at Qom in Iran. In the absence of co-operation by the Iranians, a new regime of sanctions will be imposed with the potential backing of Russia.

Foreign secretary David Miliband spelled out what was at stake when he said the UK was “100% committed to finding a diplomatic solution”, although he immediately refused to rule out the option of military action. The accusations against Iran that it was violating UN rules by covertly building a plant to enrich uranium was made at the G20 summit in Pittsburgh, US.

Obama, alongside Brown and French president Nicolas Sarkozy, completed the trio of accusers. Brown, in the arena he looks most comfortable and where the negative headlines of failed domestic policy and weak leadership look far from troubling to him, told his G20 audience that the US, France and the UK were “at one” on the issue of how they would deal with Iran.

Brown said there was “no choice but to draw a line in the sand”. He added: “Iran must abandon any military ambitions for its nuclear programme.” Two days after Brown’s rallying call to his party, in a speech where he is expected to emphasise that Labour’s financial rescue policies for the recession were copied throughout the world, Britain will be represented in talks in Geneva where Tehran will be grilled on nuclear compliance by other members of the UN security council – the US, Russia, China and France – with Germany also attending.

As one former cabinet minister put it: “At this time David Cameron will be thinking of Manchester and his party’s annual gathering. Gordon Brown will be engaging directly in a process that is trying to ensure the future safety of the planet. That is an uncomfortable backdrop for any would-be prime minister.

“Cameron will have no other choice but to lessen the focus on domestic issues and tell us what he would do on this global issue. And there lies a danger. Brown’s global credentials are already proven: Cameron could look untried, untested, inexperienced. This is an unexpected and unplanned opportunity for Gordon – and one he can’t afford to waste.”

The talks in Geneva will directly accuse Tehran of breaking up to four UN Security Council resolutions, all of them directly related to Iran’s nuclear enrichment, reprocessing and other activities related to “nuclear weapons delivery systems”.

In 2003 Iran agreed on a “subsidiary arrangement” in which it was required to inform the IAEA of any nuclear facility being designed or planned. President Ahmadinejad says “there is no secrecy”, and that the space for 3000 centrifuges, central to the enrichment process, add up to only a “pilot plant”.

Although a signatory state of the nuclear non-proliferation treaty, Iran has hidden installations before. The existence of Iran’s first enrichment plant at Natanz was only confirmed after intelligence emerged from exiled Iranians. Britain’s intelligence agency, MI6, working alongside their US counterparts and with access to some Israeli intelligence, are said to have known of the “hidden” facility at Qom for some time.

Obama is said to have been briefed on Qom by the US National Intelligence Agency between winning the presidency and entering the White House in January. Only when both US and UK intelligence suspected that Iran knew the plant’s inner secrets had been compromised was a decision taken to confront Ahmadinejad and use the global audience of the G20 to announce what Iran would be asked in Geneva.

Geneva and Brighton are not usually twinned. But Brown and Labour, still with double-digit distance between itself and the Tories in polls, now will not arrive on the south coast in such a dejected mood.

Cabinet sources says there is a wide acceptance that the threat of any challenge to Brown as Labour leader has passed. Only if Brown chose to go would Cameron face someone else in May. Brown has no intention of going. While Mandelson admits Labour is “fighting for our very lives” and Peter Hain says that plenty in the party ranks are “sullen” and reluctant and embarrassed to campaign, there is one core belief about Brown that continues to boost their optimism about the general election, despite all the opinion polls.

Prescott’s summary of Brown may yet be the anthem that dominates any underlying hope that could emerge from Brighton. Hain says the British electorate have yet to commit to Cameron. The recession’s continuing fall-out and rising unemployment have shredded much of Labour’s hopes that they would, in time, be rewarded for a return to growth. There is no indication that positive, measurable green shoots will arrive in time.

But even though there are question marks over Brown, will the Tories be trusted any better in the process of seeing jobs return? Will Cameron be trusted on pensions, mortgages, schools, the bread and butter of domestic policies? At the beginning of the recession, Brown told his party’s annual gathering: “This is no time for a novice.” That line will not be repeated by Brown in Brighton. But others will repeat it for him.

One senior Labour source said: “Iran is a reminder that government is a game where experience counts, where international reputations and authority can make the key difference between success and failure. Gordon Brown, despite the negative headlines, remains one of Labour’s greatest assets. We are facing an international crisis. Ask around. Substance matters.”

That accusation, aimed directly at Cameron’s inexperience, may yet be Labour’s core message as the election campaign approaches. It may, however, be easy enough for Cameron to counter.

Take a look at the three leaders who accused Iran of deception at the G20 and it is Obama who is the least experienced, and yet it was he who orchestrated the united response of the six main international powers as they head for Geneva.

Obama’s decision to do things differently and focus on diplomatic deal-making is said to have persuaded Russia to change its attitude to sanctions against Iran. President Medvedev was one of the leaders Obama met in one-on-meetings at the UN, while British diplomats were frantically demanding the same face-time for Brown.

Brown may have feared he would ride into Brighton this week on the equivalent of a pony and trap, rather than a chariot. But the Iranian crisis may have emphasised Prescott’s believe that a “global giant” cannot easily be dismissed.