Kieran Tierney felt he had to take a step forward in his career and move to Arsenal after living his dream at Celtic.
Celtic insisted they did everything possible to keep the left-back before selling him for £25million.
The 22-year-old had been at his boyhood heroes from the age of seven and was under contract until 2023 but, after making 170 first-team appearances and winning 10 trophies, his ambitions changed.
Tierney told www.arsenal.com: “It was the hardest decision in my whole life that I’ve had to make.
“I had to speak to my family and my close friends and my agent as well because it’s a hard decision to leave a club that you love so much. People need to know that as well.
“I feel Arsenal is a massive club and it’s a great opportunity for me as well. I’m delighted to be here.
“When I was younger, it was always Celtic. The ambition was Celtic and that was my dream when I was younger. But when you get older… I’ve done that, I’ve made my dream, I’ve lived my dream and I’ve loved every single minute of it.
“Now I just feel was the right time to take a step on. I feel this is a great opportunity for me.”
Tierney later issued an emotional and lengthy message to Celtic and their fans, some of whom have expressed their disapproval over his exit.
Addressing supporters on Twitter, Tierney said: “I tried my best to represent you every time I went onto that pitch. The relationship we had together and memories we created and history we made was incredible… I will cherish every single second of it.
“I hope we can continue to have a relationship although I understand that will never be for some people which is heartbreaking for me but I understand the emotions surrounding the situation.”
The Ladbrokes Premiership champions confirmed the club-record fee to the stock exchange after Tierney became the most expensive Scottish footballer.
A statement on Celtic’s official website read: “We did all we could to keep Kieran at Celtic. While the club did not need or wish to sell Kieran, this was an opportunity which Kieran very much wished to pursue and with the club having received a significant offer, we have agreed to the transfer.
“Kieran has been a big part of the club for a number of years, coming through our academy structure and becoming a key first team player of real quality.
“He has made a significant contribution to Celtic and we thank him very much for all he has given to the club and, of course, we wish him very well for the future in his career.”
Manager Neil Lennon added: “This is an opportunity Kieran wanted to take and while we will be sad to see him leave the club, we wish Kieran all the very best for the future.
“Although he is still a young guy, Kieran has given the club so much. He’s a great lad and I’ve really enjoyed working with him.
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“I’m sure he will continue to progress and develop and make a big impact in the English Premiership.
“It has obviously been a long process, but now it has been finalised we can now look ahead and concentrate on continuing to build our own squad and meet the many challenges ahead, domestically and in Europe.”
Lennon will step up attempts to sign a new left-back before the Scottish transfer deadline on September 2.
Summer signing Boli Bolingoli had some shaky moments in Romania on Wednesday as Celtic drew 1-1 with Cluj in the first leg of their Champions League qualifier.
Lennon said: “We have new players coming in defensively, two new full-backs and Christopher (Jullien) coming in so it’s a work in progress.
“But they have a very good goalkeeper (Scott Bain) behind them and he made a terrific save at an important part of the game.”