Northampton and England hooker Dylan Hartley has retired from professional rugby after failing to recover from a long-standing knee injury.
The former England captain, 33, has not played since December, missing his country's run to the World Cup final.
Hartley won 97 Test caps and played 251 times for Northampton over 14 seasons.
"The last few months have been difficult mentally and physically as I've come to terms with the fact that I am no longer able to compete," he said.
"I am extremely proud of my journey, both with Saints and representing England, but now is the right time to hang up my playing boots."
Only Jason Leonard has played more times for England than Hartley, who skippered them to a Six Nations Grand Slam in 2016, while they also won the Six Nations title the following year.
In addition, Hartley guided the Red Rose to series victories in Australia and Argentina in 2016 and 2018 respectively.
He was never capped by the British & Irish Lions, with a red card in Northampton's 2013 Premiership final defeat by Leicester costing him a place on the summer tour of Australia.
"My career wasn't perfect, but I wouldn't have had it any other way," he said.
"I'm privileged to have experienced some amazing highs while there have also been some personal lows, all of which are powerful experiences that will stay with me forever.
"The final chapter of my career was supposed to go a different way, but that is the nature of professional sport."
Hartley was a controversial choice as England captain, having been banned for a total of 54 weeks for offences such as gouging, biting and striking when he took the armband from Harlequins flanker Chris Robshaw before the 2016 Six Nations.
England boss Eddie Jones stood by him despite criticism of Hartley's leadership during Northampton's dismal run in 2017-18.
"Dylan has had a significant international career playing for his country having played 97 Tests, and was a tough, enduring character for us," Jones said.
"He was a foundation captain and we owe him a lot for his contribution to the making of this team. We will be forever indebted to him for his dedication and commitment to the team and his love of English rugby."
'Northampton provided me with purpose'
Hartley joined Northampton's academy in 2005 and became captain in 2009, at the age of 23.
His first six-year stint as skipper remains the longest in the club's history, and saw Saints win their only Premiership title, two European Challenge Cups and an Anglo-Welsh Cup.
"I have loved my journey in rugby. I came to England as a teenager hoping to get a few games of rugby and to see the world," said New Zealand-born Hartley.
"I could have never have predicted that one day I'd play 14 years for such a special club and go on to represent and captain England."
Hartley lost the Saints captaincy in 2015 before getting it back two years later, but he stepped away from the role in September.
The 2017-18 campaign was particularly difficult as Jim Mallinder was sacked as Northampton director of rugby after more than 10 years in charge, with the club languishing 10th in the Premiership.
"Northampton Saints has been more than just a club to me," Hartley said.
"It has been a place that has provided me with direction, purpose, a sense of family, home and belonging; and ultimately a community that I was so proud to represent every time I got a chance to play for Northampton."
He has been offered an ambassadorial role with Saints, who top the Premiership after three games this season under the stewardship of Chris Boyd.
"Dylan has been a pillar of Northampton RFC since his arrival at Franklin's Gardens in 2005," said club chairman John White.
"His achievements wearing the Red Rose of England are also almost unparalleled by his peers, but it is not only on the field where Dylan has conducted himself with dignity and humility.
"Dylan has therefore become a role model for countless young people here in Northampton, and we hope he will continue to do this."
Poor discipline created a great leader - Newman
Lennie Newman had a 21-year involvement with Northampton and was the club's team manager when Hartley joined Saints 14 years ago.
"First and foremost you have to say how sad it is because this is not a guy who's at the end of his career age-wise," Newman told BBC Radio Northampton.
"He had some very difficult times at the club in past regimes where we knew that Dylan wanted things to happen and couldn't necessarily impact them.
"But he kept his head high and was the fulcrum for that team to do some very special things."
Hartley had several disciplinary problems during his career for club and country, which arguably held him back from further success, but Newman said those experiences shaped him.
"I think it did tarnish him at the time but I think it also made him into the person that he is now," he added.
"They also created the man that we know now as one of the greatest rugby leaders that we've seen in this country."
'Hartley will go down as Saints great' - analysis
BBC Radio Northampton sports editor Graham McKechnie
For many seasons Saints were forged in Hartley's image. He was the dominant figure at Franklin's Gardens and his eight years as captain was a period of unprecedented success for the club - the highlight being the Premiership final win in 2014.
He arrived at Saints as a rather wild 19-year-old from Worcester and was made captain aged just 23. Over the years he matured on and off the pitch - the sending off in the 2013 Premiership final in particular saw a less abrasive Hartley emerge.
He lost none of his edge on the pitch but his aggression was better controlled. When Eddie Jones appointed him as his captain in 2016, when England were in disarray, it may have been a surprise to many, but not for followers of Saints. If anyone was to pull that team together it would be Hartley.
It's sad that injury has forced him to retire, but Hartley would be the first to acknowledge that as a club Saints have already moved into a new era. Yet his legacy is assured - he will go down as one of their greatest of all captains.