David de Gea announced his exit from Manchester United on Saturday after 12 years as the club's number one goalkeeper.
The Spaniard last season overtook Peter Schmeichel to record the most clean sheets in the club's history and was crowned players' and fans' player of the year four times.
But a string of high-profile errors towards the end of the campaign drew criticism and the 32-year-old struggled to adapt to manager Erik ten Hag's preferred style of playing out from the back.
In a message on social media, De Gea thanked United fans for their support, but said: "Now, it's the right time to undertake a new challenge, to push myself again in new surroundings.
"Manchester will always be in my heart, Manchester has shaped me and will never leave me."
De Gea's deal expired at the end of June, with reports the player had agreed a contract extension only for the club to withdraw that offer and come back with reduced terms.
The Red Devils are now set to move for Inter Milan keeper Andre Onana, who previously played under Ten Hag at Ajax.
De Gea made 545 appearances and kept 190 clean sheets after being signed by Alex Ferguson in 2011.
His departure means there are no players left at Old Trafford who have won the Premier League -- the last title success was in Ferguson's final season in charge, in 2012/13.
De Gea also won the FA Cup in 2016, two League Cups and the Europa League.
- Madrid move collapse -
The Spaniard often shone despite United's long slump in the post-Ferguson years.
His personal trophy haul could have been much greater but for the late collapse of a move to Real Madrid in 2015 due to paperwork not being completed before the transfer deadline.
"I would like to express my unwavering gratitude and appreciation for the love from the last 12 years," he said in his message.
"We've achieved a lot since my dear Sir Alex Ferguson brought me to this club.
"I took incredible pride every time I pulled on this shirt, to lead the team, to represent this institution, the biggest club in the world was an honour only bestowed upon a few lucky footballers."
Ten Hag thanked De Gea for his service and performances during his first season at the club after he picked up the Golden Glove for most Premier League clean sheets.
"It takes great quality and character to reach the level of even playing one game for Manchester United," he said.
"To do it 545 times over 12 years is a special achievement, particularly in the goalkeeping position where every game puts you in the spotlight.
"To have won player of the year awards from both the fans and his team-mates, each on four occasions, shows the level of his performance and he will always be remembered as one of the very best goalkeepers in the history of the club."
But despite an impressive season overall, De Gea's campaign ended on a low note.
He was judged to be at fault for the decisive goal in United's 2-1 FA Cup final defeat to Manchester City and in a Europa League quarter-final exit to Sevilla.
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