Declan Rice said his desire to play at the "very highest level" lay behind his decision to leave West Ham for Arsenal in a transfer that is set to make him the most expensive English footballer in history.
The 24-year-old England midfielder explained on Saturday why he was leaving in an open letter to the club's fans.
The Hammers also confirmed his exit on Saturday although London rivals Arsenal have still to confirm the deal, for a reported 105 million pounds (122.2 million euros, 137.5 million dollars) fee, has gone through.
Rice's final act as a West Ham player was to become only the third captain in their history, behind England great Bobby Moore and club legend Billy Bonds, to lift a major trophy following last month's Europa Conference League victory over Fiorentina.
West Ham tried to keep him but the lure of Champions League football at Arsenal proved too strong.
"I want you to know how tough a decision it has been for me to leave an environment that I have loved and cherished so much," Rice said in his letter to the Hammers' faithful.
"This club and its supporters will always be in my heart, and forever a part of who I am.
"Ultimately, though, it has only ever been about my ambition to play at the very highest level of the game."
West Ham joint-chairman David Sullivan said last month that the club had an agreement with Rice to allow him to leave in the pre-season transfer window after the player rejected the offer of a lucrative new contract.
"I am sorry to see Declan leave us but I believe that everyone at West Ham United should be very proud of the part we have played in his journey from the Academy of Football at Chadwell Heath to becoming the most valuable young player in English football," Sullivan said in a statement.
"I would like to make it clear to our supporters that we did not want to sell Declan. We wanted to build our team around him and made a series of improved, long-term contract offers to secure his future."
Rice made 245 appearances for the club and bowed out by helping them win a first major piece of silverware since the 1980 FA Cup.
Hammers manager David Moyes said: "I'd like to personally thank Declan for everything he has done during his time at West Ham United.
"Obviously it is sad when we say goodbye to someone who has been with us for so long but we must now look forward."
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