Liam Shaw told Sheffield Wednesday a move to Celtic was "impossible" to turn down as the Owls vowed to investigate his pre-contract agreement.
The Yorkshire club announced on Wednesday that the 19-year-old midfielder, who came through the Yorkshire club's academy, will move to the Scottish Premiership in the summer.
Wednesday's statement on their official website also claimed they were "investigating this matter" and said Celtic had gone quiet on a proposed coordinated release.
Speaking on his official Twitter account, Shaw said: "First of all, I would like to thank everyone at Sheffield Wednesday – the owner, all the staff, my team-mates and especially the supporters.
"After discussing the options with my family, I have decided to take on a new challenge and so, I've agreed to join Celtic, one of the most famous and prestigious clubs in the world, on July 1.
"Until the last kick of the ball of the final game of this season, I will continue to give my all for Sheffield Wednesday and to help my team-mates.
"I work hard every day in order to make my family proud and I'll always give everything in training and in games.
"I'm determined to contribute as much as possible to the team, so we have a successful end to the season.
"Sheffield Wednesday is a massive and brilliant club and I'm extremely grateful for everything the club has done for me.
"The club will always be special for me as I grew up a fan and nothing can change that for me.
"In the end, I based my decision purely on the football aspects.
"By joining Celtic next season I will fight for a place in a team challenging for titles, trophies and to play in Europe, which is something I've always dreamed of.
"I am determined to play at the highest level I can possibly achieve in my career.
"I thank Celtic for this amazing opportunity, which is impossible for me to turn down."
Wednesday's statement read: "Sheffield Wednesday can confirm that midfielder Liam Shaw has signed a pre-contract agreement with Scottish Premiership side Celtic.
"We are currently investigating this matter and will consider if there are any further steps available to protect the club's interest.
"It was not our intention to place this development into the public domain out of respect to all parties concerned.
"However, we were contacted by Celtic informing the club of their intention to formally confirm this development, which was revealed by the former Celtic manager in a media briefing.
"Both clubs exchanged statements in advance of a coordinated release but Celtic have since ceased correspondence to this end.
"Shaw remains a Sheffield Wednesday player until the expiry of his current contract and will be available for selection for the remainder of the season.
"We will make no further comment at this time."
Celtic also welcomed Shaw in a Tweet which read: "We are delighted to confirm that we have finalised a pre-contract agreement with Liam Shaw."
