After a decade as chancellor -- during which the two men's relationship has reportedly soured from tolerant alliance to poisonous rivalry -- Brown will become prime minister, after replacing Blair as Labour leader Sunday.

Since being named to succeed Blair last month the 56-year-old Scot has battled to shed his uncharismatic image -- aides say he is genial in private -- but he will have his work cut out after the media-friendly Blair.

James Gordon Brown was born to a Presbyterian Scottish pastor's family on February 20, 1951. At the age of just 16 he went to Edinburgh University, where he gained a first-class degree in history followed by a doctorate.

He worked as a politics lecturer at his alma mater and Caledonian University, before moving on to Scottish TV, a franchise of Britain's biggest commercial broadcaster, ITV.

During those early years, he forged a strong link with the Labour Party, first offering to canvass for them in a by-election when he was 12, and later joining the party officially at 18.

Brown lost his initial attempt to join parliament in 1979, but was elected to represent Dunfermline East in 1983 -- the same year Blair became an MP -- and went on to hold various posts in Labour's 18 years of opposition.

Sharing an office in parliament, as well as a desire to modernise the party with Blair, Brown has been described as the man who tutored Blair in national politics.

In 1992, after Labour lost a fourth successive election, he was named Labour's finance spokesman; at the time, he was mentioned, along with Blair, as one of then leader John Smith's potential successors.

Two years later, though, Smith died and according to a popular legend, Brown and Blair made a deal in a north London restaurant whereby Brown would back Blair's leadership bid, and Blair would stand down in favour of Brown as little as four years into a Labour government.

Following Labour's 1997 election victory, Brown was named Chancellor of the Exchequer (finance minister), and within a week he gave the Bank of England, Britain's central bank, full independence to tackle inflation.

In the past decade, in which he has become the longest-serving finance minister without interruption, he has also championed the cause of forgiving developing country debt.

His detractors, however, chide him for his controlling nature -- a former top civil servant described him as "Stalinist" and a cabinet colleague said he was a "control freak".

Brown's credibility in managing the economy has also been hurt by a string of recent scandals involving the country's state pension fund, while his commitment to fiscal prudence, which earned him the "Iron Chancellor" nickname, has been drawn into question.

The chancellor is reportedly more of an Atlanticist than a Europhile -- when Blair wanted to lead Britain into the eurozone, Brown effectively vetoed the decision.

He has also acknowledged that mistakes were made in Blair's biggest foreign policy adventure -- Iraq -- though he has been broadly supportive of Britain's military actions in Afghanistan and Iraq.

Brown has steered Britain to an unprecedented era of sustained growth, high employment, and low interest rates, all despite suffering personal setbacks in recent years.

In January 2002, his first child Jennifer was born prematurely and died 10 days after her birth. He has since had two boys, though the second was diagnosed with cystic fibrosis in November, aged just four months.

Through it all, though, it has been his relationship with Blair that has grabbed the headlines and captured the country's attention, with tensions rarely far from the surface.

The chancellor was alleged to be the mastermind of an attempted political coup last September when eight junior members of government resigned in a matter of days, calling for Blair to step down -- he denies the charge.