The news will come as a significant setback for England's World Cup hopes given Williamson was set to lead the Lionesses into this summer's tournament.
Williamson captained England to their historic Euro 2022 triumph and has established herself as an instrumental part of Sarina Wiegman's team in the heart of defence, but the knee injury she sustained while playing for Arsenal at Manchester United on Wednesday has ruled her out for the 2023 Women's World Cup.
"Until I have the words to express my feelings properly I will struggle to verbalise them. The noise around the situation is loud and I need some quiet to let it all sink in," she wrote in an Instagram post on Friday.
"Unfortunately the World Cup and Champions League dream is over for me and everyone will think that's the main focus, but it's the day to day of what I'm going through that's the most draining of my thoughts.
"I had tears in my eyes and made peace with it the night it happened and since then I have been following the steps I'm told to in order to best help myself in the short and long term.
"Ultimately, I think it's just my time. In the past couple of years alone I have watched teammates beat serious illnesses and adversity with the biggest of smiles on their faces. I also had the perspective that globally there are much greater difficulties and therefore my circumstances right now are just that, circumstantial, and I've seen a lot worse.
"I haven't had a day since last October when I've walked on to the pitch without a physical or mental mark over me, and that's professional sports. So now I have to listen to my body, give it what it needs and if everything happens for a reason, then we'll see what road this turn sends me down.
"I have given and will continue to give everything my body, mind and heart possibly has to Arsenal and Lionesses, I will still be there through thick and thin for all my teammates and their biggest supporter. I feel your love and support, so thank you! All I ask is for a little bit of time and space to deal with all that is to come."
Williamson picked up the injury in the 15th minute of the match as she tussled with Katie Zelem for the ball. There was minimal contact but Williamson landed awkwardly on her right leg.
Arsenal said in a statement: "Leah will now begin a period of rehabilitation and is set for an extended spell on the sidelines. She will undergo surgery in due course.
"Everyone at Arsenal will be supporting Leah closely throughout the journey ahead and we would ask that her privacy is respected at this time."
With Williamson absent, England will need to appoint a new captain for this summer's tournament with her defensive partner Millie Bright expected to be a front-runner for the role. But Bright is also sidelined with a knee problem as Wiegman deals with what is threatening to descend into an injury crisis ahead of the tournament.
Euro 2022 Player of the Tournament Mead needs a "miracle" to be fit to play in the World Cup, according to Wiegman, as she continues her recovery from an ACL injury she picked up in December. Alongside Bright, her Chelsea teammate Fran Kirby is also recovering from a knee injury she sustained in January.
So while England remain hopeful both Bright and Kirby will be fit, the news of Williamson's injury means they will look set to be without two of their main stars at the World Cup. With Jill Scott and Ellen White both retiring post-Euros, it will be a new-look England side heading to Australia just a year after they won the Euros.
Williamson's injury means England will require a new partnership at the heart of their defence. She partnered Bright through the Euros so one option could be to shift Alex Greenwood to centre-back, or Wiegman could look to Esme Morgan who started in England's 2-0 defeat against Australia in Bright's absence.
While they have other options like Lotte Wubben-Moy it could also open the door for a return for Steph Houghton, England's former captain who is yet to feature under Wiegman.