Harry Maguire says he has been stripped of the Manchester United captaincy by manager Erik ten Hag.
Maguire was appointed skipper by Ten Hag's predecessor Ole Gunnar Solskjaer in January 2020 - five months after signing for £80m from Leicester City.
The England centre-back says he is "extremely disappointed" but will "continue to give my all" in the shirt.
The 30-year-old made eight starts in the Premier League last season and is linked with a move to West Ham.
United said Ten Hag will announce a new captain "in due course, after he has informed the players". The new captain is unlikely to be confirmed until United arrive in New York on Thursday morning for their pre-season tour.
"Everyone at Manchester United thanks Harry for his contribution as captain over the past three and a half years," the club added.
Maguire played in 31 of the club's 62 matches in all competitions during the 2022-23 season, with Portuguese midfielder Bruno Fernandes wearing the armband in his absence.
His contract at Old Trafford runs to 2025.
"After discussions with the manager today he has informed me he is changing captain," Maguire posted on social media.
"He outlined his reasons to me and whilst I'm personally extremely disappointed, I will continue to give my all every time I wear the shirt.
"I wanted to say a massive thank you to the Manchester United fans for all their brilliant support whilst I've been wearing the armband.
"Since the day I took on the role, three and half years ago, it's been a huge privilege to lead Manchester United and one of the proudest moments of my career to date.
"I've done everything I possibly could to help United be successful - on and off the field. I will always be grateful to Ole Gunnar Solskjaer for first giving me the responsibility and I wish whoever now takes it on every success."
Raphael Varane and Lisandro Martinez were favoured over Maguire at the heart of United's defence last season, and when the latter was injured, Victor Lindelof deputised.
England left-back Luke Shaw was also deployed as a centre-back on several occasions by Ten Hag.
Former United boss David Moyes is keen to make additions to his West Ham squad following the sale of skipper Declan Rice to Arsenal, which could generate £105m.
Moyes is interested in Ajax's Mexican midfielder Edson Alvarez as a possible replacement for Rice, but Maguire could prove to be a useful addition at the back, where both members of West Ham's first-choice pairing - Kurt Zouma and Nayed Aguerd - suffered injury issues last season.
Analysis - 'Maguire won't go quietly'
BBC senior football reporter Simon Stone
This announcement was fairly obvious given how last season went.
Ten Hag stood by Fernandes in the wake of his heavily criticised behaviour during the 7-0 defeat by Liverpool.
Speaking to BBC Sport before the FA Cup final, Fernandes said he wanted to repay Ten Hag for his trust, which will almost certainly extend to being given the armband on a permanent basis.
Amid interest from other Premier League clubs, sources close to Maguire have said the defender is happy at Old Trafford.
That happiness has now been tested.
It is clear Ten Hag is willing to let Maguire leave. It seems obvious Maguire won't go quietly.
Now the question is whether the England man is selected for a four-match tour of the United States which the club will embark on immediately after Wednesday afternoon's friendly with Lyon at Murrayfield.