Jude Bellingham has signed for Borussia Dortmund from Birmingham for an initial fee of £25m.
The 17-year-old English midfielder completed a medical in Germany on Thursday after Dortmund persuaded him to move to the Bundesliga club over Manchester United, who had also made him an offer.
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It is understood Birmingham will receive a guaranteed £25m up front, with fees worth 'several millions more' potentially added on depending on appearances, achievements and milestones.
Bellingham - who Dortmund say has signed a "long-term contract" with the club - said: "I can't thank Birmingham enough for what they have done for me, not just this past year but since the age of seven.
"Of course I wish everyone associated with the club nothing but the best. Once a blue always a blue.
"However, I'm incredibly excited to have joined one of Europe's biggest clubs. The direction they are heading in and how much they help to improve young players made it an easy decision for myself and my family.
"I can't wait to perform in front of all the fans when they are back inside one of the most beautiful stadiums in the world."
Bellingham - who only signed a professional deal with Birmingham this month - follows in the footsteps of Jadon Sancho, who moved to Dortmund in 2017 and has since gone on to become one the stars of the Bundesliga and an England international.
Dortmund courted the midfielder, who turned 17 in June, for more than two years, amid stiff competition from most of Europe's top teams, including Chelsea, Real Madrid, Bayern Munich and, most notably, Manchester United.
United pulled out all the stops in their attempts to sign the box-to-box midfielder, offering him a tour of their training ground with Sir Alex Ferguson.
But the England U17 skipper has chosen Dortmund due to the club's long history of developing young talent and their policy of giving them the chance to play in the Bundesliga.
Dortmund's sporting director, Michael Zorc, confirmed Bellingham will be part of their first team, saying: "Jude Bellingham confidently chose BVB and, of course, was primarily concerned with the sporting perspective that we showed him.
"He has enormous potential that we want to develop with him in the coming years. He already has a surprising amount of quality in his own possession as well as against the ball, and on top of that, the player also has a strong mentality.
"We immediately see Jude as a reinforcement for our professional squad, but of course we will also give him the time to get used to the higher level."
Bellingham will be available for Birmingham's final game of the season, against Derby on Wednesday, and will then join Dortmund for pre-season training on July 30 and a training camp in Switzerland on August 10.
Bellingham - who will wear the No 22 shirt in Dortmund - has been sought after in England and in Germany even before he became Birmingham's youngest ever first-team player at the age of 16 years and 38 days last August.
Analysis: Why Bellingham is a special talent
Sky Sports' Adam Bate:
Bellingham made his debut for the England U15 team at just 13 and clubs all over Europe have been targeting him for years. Now it is Borussia Dortmund spending big on Jude Bellingham.
At the age of 17, his transition to the senior game has been seamless. The touch and the class are apparent but he has coped physically too - putting in plenty of tackles - and handled the mental side of the step up impressively well. All the signs are that he is someone who is going to maximise his talent - and that maximum looks higher than most.
Birmingham were well aware they had a unique talent on their hands for some time and the club deserves huge credit for their handling of Bellingham's development both before and after his breakthrough into the first team. His transition has not been by luck. It has been a carefully thought-out process in order to help him along each step of the way.
The question now is whether he is prepared to take the next step in his development. There will be those who fear it is too soon. He has been playing regular Championship football, after all.
But the next step for Bellingham is the Bundesliga, and Dortmund were not the only ones convinced that this is a player who is ready for that step sooner rather than later.
Sky Sports News' North West reporter James Cooper:
Ultimately, it came down to a decision between Manchester United and Borussia Dortmund.
In their attempts to sign the teenager, United rolled out the red carpet for Bellingham and his family, who were impressed by their tour of the Aon Training Complex at Carrington, which featured an audience with Sir Alex Ferguson.
United coaches and recruitment staff had been monitoring the midfielder's development for almost two years and, in doing so, built a close relationship with his family and their coaching counterparts at Birmingham City, who have forged a reputation for producing impressive young players.
Birmingham and Bellingham could have made more financially from the player staying in England and relocating to Old Trafford but, while he would have been part of Ole Gunnar Solskjaer's squad next season, the competition for places would have been fierce.
Next season the priority for Manchester United is getting themselves back in the hunt for a Premier League title and, while youngsters are part of their DNA, as demonstrated this season, the first team is all about players who can deliver success now.
Jude Bellingham may have chosen not to move to Manchester United at this stage of his young career but United's pursuit of him and the admiration they have shown him may stand them in good stead further on down the line.