England left-back Luke Shaw became the most expensive teenage footballer in history after joining Manchester United from Premier League rivals Southampton in a four-year deal worth a reported £30 million (51m, 38m euros).

That figure surpassed the £25.6m fee United paid Everton for Wayne Rooney in 2004.

"Manchester United is delighted to announce the signing of Luke Shaw from Southampton Football Club," a United statement said.

The 18-year-old Shaw, whose contract contains an option for a further year, signed for United on Friday.

His arrival at Old Trafford meant United had announced an estimated £60m of spending in two days after completing their first signing since Louis van Gaal, currently coaching the Netherlands at the World Cup, became their manager after midfielder Ander Herrera joined from Athletic Bilbao for a reported £28.5m on Thursday.

"I am thrilled and excited to be joining Manchester United," the teenager said after passing a medical in a statement on United's website.

"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," added Shaw, who made 67 appearances for the south coast club since his debut in 2012.

But he said he'd no hesitation in joining United.

"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."

Shaw won the third of his three England caps in the goalless draw with Costa Rica that saw Roy Hodgson's men, already eliminated following defeats by Italy and Uruguay, conclude their disappointing World Cup.

He was the youngest player to feature at Brazil 2014 thus far.

"Luke is a very talented young left-back with great potential" said United assistant manager Ryan Giggs.

"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."