Jessica Ennis-Hill, who won heptathlon gold for Great Britain at the London 2012 Olympics, has retired from athletics.
The 30-year-old had hinted at retirement after winning a silver medal at the Rio Olympics in August.
In a post on social media, Ennis-Hill - also a double world champion - said it was "one of the toughest decisions I've had to make".
"I've always said I wanted to leave on a high and have no regrets," she added.
Ennis-Hill's heptathlon gold was one of the most iconic moments of London 2012's 'Super Saturday'.
But she missed out on retaining her Olympic title in Rio by 35 points to Belgium's Nafissatou Thiam. Speaking afterwards, she said she would not rush a decision over ending her career.
In her statement on Thursday, she said "retiring now is right".
"I want to thank my family and incredible team, who have spent so much of their time supporting me and enabling me to achieve my dreams," she added.
"Also, a huge thank you to all those people who have supported and followed my career over the years."
After winning Olympic gold in London, Ennis-Hill had her first child, Reggie, in 2014 and won a second world title just 13 months later.
Former British sprinter and Olympic gold medallist Darren Campbell told BBC Radio 5 live: "It's a sad day but it's also a day where we should celebrate what a phenomenal athlete Jessica Ennis-Hill was. She gave so much to sport, achieved so much, so many inspirational moments throughout her career.
"She is a normal girl, she has achieved things that are exceptional and mind-blowing. It is only when you sit back and reflect that you realise how mind-blowing they are."
BBC athletics commentator Brendan Foster described her as "one of the all-time British greats".
He added: "She was carrying the whole nation on her shoulders going into London 2012 and the pressure on her was immense, like nobody else. But she delivered an indelible memory which we shall never forget.
"She has been a pleasure to watch and to be around. The sport will miss her, the British public will miss her but I'm sure she is doing the right thing.
"It's a sad day for the sport - but a great day for her."
Former British heptathlete Kelly Sotherton told BBC Sport she thinks Ennis-Hill "will love not having to go to the track".
She added: "Two years ago she had her first child and she is a very family-oriented person. She's quite private, she's not flash, she wants to have more children. So I think she will slot into her life outside of the sport.
"I hope she continues to play a part in sport, because she has a lot of great attributes that she can share to help future generations."
Foster also believes she will have a future role to play in athletics.
"The sport will be foolish not to keep her on at some level, she is a huge asset," he said.
"She has been up, she's been down, she has won, she's lost, but at every point she has conducted herself magnificently.
"Everyone says she is a great role model for women - and she is - but she has been a role model for men, too. To look at her, she is so tiny standing next to many international athletes, but inside that smiling exterior she has internal steel. Male and female athletes think the world of her."
Ennis-Hill's coach Toni Minichiello said she was "one of our sporting greats", and the manner of her retirement - "walking out of the stadium by stepping off the podium" - was "fitting".
"We've known for a long time this day was coming. Many sports people hold on too long," he added.
"Despite all the fame and money she's never forgotten where she's come from - most of her friends she's had from school days. She's humble, she grafts, she pushes herself hard and she never gives up."
British Athletics described her record as an athlete as "phenomenal", adding: "And that's without considering the challenges of returning from pregnancy to win world gold and Olympic silver."
Retirement means she will not return to the stadium where she won Olympic gold for the World Championships in London in August 2017.
What she won
Ennis-Hill's first medal came at the 2006 Commonwealth Games in Melbourne at the age of 20, when she took bronze behind fellow British athlete Kelly Sotherton.
She then missed out on the medals at the 2007 World Championships and had a year out with injury - missing the 2008 Beijing Olympics - before beginning a period of domination with gold at the 2009 World Championships in Berlin.
She followed that up with another gold at the 2010 European Championships in Barcelona, Spain and then won silver at the 2011 Worlds in Daegu, South Korea before heading to her home Olympics in London in 2012 as an overwhelming favourite.
She delivered gold on 'Super Saturday', comfortably winning in a British and Commonwealth record.
After missing the 2014 season to have her son, she returned to win her second world title in Beijing in 2015 and then finished second at the Rio Olympics this summer.
Tom Fordyce, chief sports writer
If the timing of her retirement is no great surprise - the emergence of Nafi Thiam at this summer's Olympics moved heptathlon to a new level - Ennis-Hill's career has still been about the gloriously unexpected: winning world titles and Olympic golds when she was supposed to be too small to succeed, overcoming stress fractures that cost her early championship chances, beating women once banned for doping and others who were only retrospectively sanctioned.
Her triumph at London 2012 was one of the iconic moments of an impossible fortnight; to then win back her world crown as the mother of a young son was arguably a greater achievement still. In a tainted era, a splendid role model. British sport has been lucky to have her.
British Athletics performance director Neil Black: "The entire performance team of coaches, athletes and support staff will miss her at events.
"Not only has she competed at the highest level, but she has been a warm and positive presence within the GB team and a great example to the younger team members."