Luke Shaw has become the most expensive teenage footballer in history after he completed his €33.7 million move to Manchester United from Southampton.

Shaw, 18, passed a medical at the club's Carrington training ground on Thursday evening and the following day he signed a four-year contract, with the option of another year.

The deal is understood to be worth £27 million, plus add-ons. That figure eclipses the £25.6 million fee United paid for Wayne Rooney in 2004.

“Manchester United is delighted to announce the signing of Luke Shaw from Southampton Football Club,” a United statement read.

England international Shaw had earlier indicated on his Twitter account that the deal was complete, changing his biography to read “Professional footballer at Manchester United Football Club”.

Shaw established himself as one of the hottest prospects in world football after coming through Southampton’s fabled academy, attracting admiring glances from many, including boyhood club Chelsea.

Despite not turning 19 until July 12th, the left back has already racked up 67 appearances for Saints and three England caps.

The defender now hopes his career can blossom at Old Trafford alongside the likes of Rooney, Robin van Persie and Juan Mata.

“I am thrilled and excited to be joining Manchester United,” Shaw said.

“I have been at Southampton since I was eight years old and I would like to thank them and the fans for everything they have done for me.

“I want to continue to progress my career and joining United is the ideal place for me to do that.

“I am looking forward to this new chapter in my life and also looking forward to learning from the world class players and management at the Club.”

The add-ons in the deal could take the figure beyond €37 million.

That may seem excessive for such a young player, but United assistant manager Ryan Giggs is sure the club have bought one of the best prospects in English football.

“Luke is a very talented young left back with great potential,” said Giggs, who will work under new manager Louis van Gaal next season.

“He has developed immensely during his time at Southampton and has all the attributes to become a top player.

“I am delighted he has joined Manchester United as I believe this is the right club for him to continue his development as a young player and eventually fulfil that promise.”

Shaw made his England debut in the friendly against Denmark in March and usurped idol Ashley Cole in Roy Hodgson’s pecking order, leading the experienced left back to call time on his international career.

Shaw was one of few positive points from a miserable World Cup campaign for England and became the youngest player to feature at the tournament when he started Tuesday’s goalless draw with Costa Rica.

Shaw will be competing with Patrice Evra for the left back role at Old Trafford after the France international signed a one-year contract extension last month.

Last season’s back-up Alexander Buttner looks set to move to Dinamo Moscow, joining Rio Ferdinand and Nemanja Vidic in leaving as the club look to kick on after a miserable season.

United limped to a seventh-place finish last term, in a season which saw David Moyes lose his job in charge with Dutch coach Van Gaal coming in as his replacement.

Van Gaal made his first United signing on Thursday when he purchased Ander Herrera from Athletic Bilbao.