Sir Paul Stephenson is to become the new Metropolitan Police Commissioner. The 55-year-old was selected ahead of the PSNI's Sir Hugh Orde, sources said.
The Scotland Yard appointment will be formally confirmed in a written parliamentary statement.
Home Secretary Jacqui Smith and London Mayor Boris Johnson agreed on Sir Paul following a final interview on Monday.
Their decision was approved by the Prime Minister before the British Queen confirmed the appointment.
The news will come as a bitter disappointment to his rival Sir Hugh Orde, the chief constable of the Police Service of Northern Ireland.
The cross-party interview panel agreed on Sir Paul to serve London and Britain for the next five years in the Stg £254,000 post.
His experiences as deputy commissioner under Sir Ian Blair since 2005 gave him a powerful insight into the challenges of the top job.
Controversies included the shooting of Jean Charles de Menezes, a string of public gaffes and an angry row with senior ethnic minority officers.
Sir Ian was ousted from his role last October within 24 hours of Mr Johnson taking control of the Metropolitan Police Authority.
Sir Paul faced tough questions just days after he took over as acting commissioner last December in the wake of the arrest of Tory frontbencher Damian Green.
The resulting furore catapulted Sir Paul into the media spotlight and gave the straight-talking officer an opportunity to show his mettle.
It also led to Mr Johnson revealing the two men clashed in private as the Tory mayor said the operation was over the top and predicted it would come to nothing.
The Met role combines regional and national responsibilities and Sir Paul will lead a staff of more than 50,000 and command a £3.5 billion budget. He must continue the fight against terrorism, lead police strategy nationwide and secure the 2012 Olympic Games.
In London, the new commissioner faces spiralling youth violence, a potential recession-fuelled crime wave and low morale as budget tightening looms large.
Two other contenders, West Midlands boss Sir Paul Scott-Lee and Merseyside chief Bernard Hogan-Howe, were earlier eliminated from the contest.
The Home Office and the Metropolitan Police Authority, which were responsible for appointing the commissioner, declined to comment.
PA