The deputy commissioner of the Metropolitan Police has been named as the force's new chief.
Sir Ian will be in charge of 30,000 police officers
Sir Ian Blair, 51, applied for the £225,000-a-year post in 1999, but missed out to current Scotland Yard chief Sir John Stevens
He has been deputy commissioner in London since 2000 and takes over the lead role in the new year.
As commissioner of the UK's largest police force, Sir Ian will be in charge of 30,000 officers.
Sir Ian said: "'It is an enormous honour to have been chosen to lead what I believe to be the greatest police service in the world.
"The Met has been through a period of substantial change in the last few years and it is an absolute privilege to be entrusted with leading, shaping and modernising the Service in the years ahead."
Sir Ian, who joined the Met in 1974, paid tribute to Sir John's achievements and said he had worked closely with him over the past five years to make London safe.
The Oxford-educated father-of-two, who has earned a reputation for being a "thinking man's policeman", said he was confident he could build on this work to create a police service to be "proud" of.
Sir John said he believed his successor was "the right man to meet the changing demands and future challenges of policing the capital".
"I know he has what it takes to lead an organisation as complex and demanding as the Metropolitan Police Service," he added.
Announcing the appointment, Home Secretary David Blunkett said Sir Ian had "demonstrated powerful leadership qualities" and had been a "key player" in police reform.
Mr Blunkett said: "He is also an exceptional officer with strong personal conviction and a high level of integrity.
"I look forward to working closely with him in tackling the challenges that face the capital and the police service as a whole."
West End Ban
In his new role, Sir Ian will also be at the forefront of the challenge of protecting the UK from terrorist attack.
In the past has called for more female, ethnic minority and gay recruits but provoked controversy in 2002 when he said society could not duck the fact that most muggers were black.
The National Black Police Association said his comments could deter black recruits.
Sir Ian has backed a plan for unaccompanied children of under 16 to be banned from London's West End after 2100.
And he wants to have 34,000 officers and 1,000 community support officers on the streets of London by 2006.