The Spanish goalkeeper, who won the Premier League Golden Glove award for most clean sheets last season, was criticised for a number of high-profile errors in the final weeks of the campaign.

His contract with United expired at the end of June, with no new deal agreed despite talks that took place throughout the season.

United have been linked with a move for Inter Milan keeper Andre Onana.

In a message on social media, De Gea, 32, 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."

When De Gea's contract expired last Friday, United said they remained in talks with the Spanish international goalkeeper.

That followed reports De Gea had signed a contract extension only for the club to withdraw that offer and come back with reduced terms.

De Gea wrote: "I would like to express my unwavering gratitude and appreciation for the love from the last 12 years. 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."

United boss Erik ten Hag thanked De Gea for his service to the club in a statement.

"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."