Declan Rice says his ambition to play at the highest level was behind his "tough" decision to leave West Ham for London rivals Arsenal.
The Hammers have confirmed their club captain's departure after 10 years, with his switch to the Gunners now comfirmed.
Rice, 24, leaves the Irons on a high after leading them to Europa Conference League glory last season, making him just the third skipper in their history to lift a major trophy.
In an open letter to fans, the midfielder said: "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.
"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.
"Playing on the opposite team to West Ham for the first time will be an unusual experience.
"I'm not sure yet exactly how I will feel, but I also know you will all understand and respect that my professional loyalties have to now lie with my new club."
Having graduated through West Ham's Chadwell Heath academy, Rice went on to make 245 appearances for the East London outfit.
He was handed the captain's armband on a permanent basis in 2022 following Mark Noble's departure.
On his Instagram, Rice added: "I can’t believe an incredible 10 years has come to an end. West Ham has been a massive part of my life on and off the pitch. Saying goodbye is never easy.
"I’ve had some unbelievable highs, none higher than that night in Prague. I’ve also grown as lot as a person, a player and as a captain.
"From the very first moment I captained the side, under the guidance of Nobes [Mark Noble], I have felt nothing other than pride and passion. As a captain, delivering that European trophy was the icing on the cake.
"My connection with the fans means everything to me and my family. You have treated us like one of your own since the day I arrived. Thank you.
"Everyone knows how special West Ham is as a club that goes for all managers, coaches, back room staff and of course, my team mates.
"Thank you for some amazing memories and some of the best days of my life."
