David Miliband is the first out of the traps in the race to replace Gordon Brown as Labour leader.

The leading Blairite has left little doubt of his ambition, though some insiders question whether he has the personal steel and the breadth of support in the party he would need to seize the crown.

Mr Miliband, 44, was a close ally of Tony Blair from the earliest days of New Labour, working for him in opposition from 1994 and heading the Number 10 policy unit during his first term in power.

Even before he became an MP, he was a key figure in reshaping the party's agenda, nicknamed "Brains" by Alastair Campbell for his powerful intellect and mastery of policy detail.

Parachuted into the safe seat of South Shields shortly before the 2001 election, he was a minister within a year and joined the Cabinet in 2005, rising to become Environment Secretary, where he put the issue of climate change firmly on the agenda for the first time.

When his former mentor quit 10 Downing Street in 2007, Mr Miliband's appointment to one of the great offices of state reflected Mr Brown's determination to mend fences with the Blairites and include talent from all sides of the party in his administration.