Phil Jones will leave Manchester United when his contract expires this summer, after a "very difficult" few years because of injuries.
The 31-year-old, who has been capped 27 times by England, made 229 appearances for United and scored six goals.
The defender joined in 2011 from Blackburn Rovers and was part of the Premier League-winning team in 2012-13.
"I've worked tirelessly hard - I literally left no stone unturned," Jones said in a club interview.
"I tried to get back as quick as I could, every time I was out in the last couple of years."
His final appearance for United came in a 3-0 win over Brentford at the end of last season, and he has been sidelined with injury during this campaign.
"I wish I could have played more. I wish I could have given more to the many squads I played amongst. I will say, from the bottom of my heart, I did everything I could," Jones wrote in an open letter to Manchester United fans.
"It's been very difficult, the last couple of years, there's no denying that," he added in his club interview.
Jones admitted he felt like he was "letting people down" and that he "found it hard to even speak to my team-mates" because he was "hurt that I couldn't help them".
"My family have been absolutely instrumental in keeping me on the straight and narrow and keeping me focused, to try to get fit and try to play more.
"I think the highlight for me would have to be the Wolves game [returning from injury last season], when I came back.
"That was just a moment that I'll never forget. It gives me goosebumps now, thinking about it. I mean I worked so, so hard to get to that stage of fitness.
"That was a huge, huge moment in my career and the fans, that day, were incredible. The love that they showed me for that game, the passion that they showed, was sensational.
"I can't thank them enough for that. They've been a constant support throughout my time here."
During his 12 years at the club, Jones won the Premier League in Sir Alex Ferguson's final campaign with United.
He also won the FA Cup in 2016, the League Cup and Europa League in 2017 and the Community Shield in 2011 and 2013.
Speaking in 2021, Jones said had "been through hell and back over the last couple of years" after struggling with injury and lockdown.
He described breaking down in tears and feelings of "rage" and helplessness over being labelled a figure of ridicule by some of his own supporters, and described once being verbally abused in the street in Hale while out with his two daughters.
Manchester United said: "Our number four is now taking some time before deciding his next move.
"Everybody at United would like to wish Phil the very best of luck for the future."
Manager Erik ten Hag said: "He has a great career, what he achieved, winning the FA Cup, winning Europa League, being present at two World Cups, massive career at Man United.
"All the credits for his career."