Saints have confirmed the sale of Luke Shaw to Manchester United.
The 18-year-old has signed a four-year contract at Old Trafford, with the option to extend it by a further year.
Saints are beleived to have received a fee of £27m, with add-ons that could take it well beyond a £30m total.
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It makes Shaw the biggest sale in Saints' history and the most expensive ever teenager in world football.
As revealed by the Daily Echo, Shaw told Saints at the end of the season that he wished to leave for United this summer, and he has got his way soon after returning from the World Cup with England.
Saints have issued a statement to say: "Southampton Football Club can confirm that Luke Shaw has today completed his move to Manchester United.
"The Saints Academy graduate moves to Old Trafford for an undisclosed fee. The fee will amount to the highest-ever transfer fee received by the club, making the 18 year-old the most expensive teenager ever.
"Executive Director Les Reed commented: “While we are sad to see Luke depart the club, we fought to ensure that we got the right deal for a player in whom we have invested a great deal of work over the past decade.
“Luke joined us at the age of eight, and our academy staff and coaches have helped him to progress into a top-class player. He will leave our club and carry forward our top-class pedigree, as others have done before.
“This deal is a good one for Southampton Football Club, and hopefully sends a clear message to other clubs wishing to bid for our players.
“Luke’s transfer fee will be reinvested into the team as our new manager, Ronald Koeman, builds for the new season and for years to come.”
CEO Gareth Rogers commented: “Luke's world-record transfer to Manchester United is yet another endorsement of the great work undertaken at our academy and by the staff at Southampton Football Club.
“We can now move forward and continue to invest in the future of our club.”
United also put a statement which included quotes from Shaw himself.
"I am thrilled and excited to be joining Manchester United," said the England left back.
"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."
Manchester United assistant manager Ryan Giggs added: "Luke is a very talented young left-back with great potential.
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."