Despite Brown's efforts to shrug off his dour image, his style in office is expected to contrast markedly with Blair's easy media-friendly manner which has charmed so many at home and abroad.

On substance, Brown is widely seen as more eurosceptic than his predecessor and more cautious in his relations with Washington, after the anger triggered by Blair's decision to become Bush's closest ally over the 2003 Iraq war.

But hours before yesterday's handover of power in London, US President George W. Bush sought to downplay Brown's reputation as overly serious, and insisted they got on well.

"Gordon came here and he wasn't the image of the dour Scotsman at all. He was relaxed. It was a good meeting," he told The Sun daily, adding: "Tony has been very gracious about Gordon Brown to me."

Brown, described by a former top civil servant as "Stalinist" in the run-up to his succession, was widely alleged to have orchestrated a political coup last year which forced Blair to agree to stand down.

But as he prepares for office he has sought to humanize his image, smiling more and changing his haircut, and promising an end to "celebrity" politics -- a barely-disguised dig at Blair's constant courting of the media.

He also sought, albeit subtly, to distance himself from Blair over Iraq, the key issue that dogged the second half of Blair's premiership.

While ruling out immediate withdrawal of British troops and vowing to "keep our obligations to the Iraqi people", he has admitted that mistakes were made and called for more focus on development and reconciliation.

Some say the "the big issue" for Brown will be his relations with Bush's successor.

"If there's a Democrat there in 18 months, the stage will be set for a renegotiation of the Anglo-US relationship," said Mark Wickham-Jones of Bristol University.

On Europe, Britain's other key alliance, Brown is expected to be cooler than Blair.

It was reportedly his influence which has kept Britain out of the single currency. New French President Nicolas Sarkozy, speaking shortly after his election last month, urged Brown not to see the European Union as "outmoded".

Last week's agreement of a new European Union treaty may however pose problems for him, with eurosceptics demanding a referendum.

He will likely resist that pressure, but the issue could dog him, in particular in the run-up to a general election which is being predicted could come as early as next year.

Britain's notoriously eurosceptic voters will be watching closely, in particular as the contest hots up between Brown and opposition Conservative Party leader David Cameron, among those demanding a public poll on the EU pact.

An Ipsos/MORI poll earlier this month asked voters who they would rather go to a pub for a drink with -- 38 percent said Cameron, 37 percent said Brown and 25 percent said they did not know.

After years holding out for the leadership, there are fears that Brown could crash and burn after a relatively short time like Anthony Eden, prime minister during the 1956 Suez Crisis, whose wait for the top job mirrors his own.

But former Labour Party chairwoman Hazel Blears, while saying Wednesday was a "bittersweet" day with Blair's departure, insisted Brown's arrival will re-invigorate the government.

"We will remember Tony Blair with affection, pride and admiration," she said.

"But today as well is Gordon Brown's day and I think that sense of Labour really renewing itself, not just in terms of the faces and personalities but also that sense of energy, dynamism," she told broadcaster GMTV.