Women's World Cup star Alex Morgan will not play again this year due to a knee injury, she has announced.

The United States forward Morgan, 30, made the announcement on Instagram, calling an end to her season with the Orlando Pride of the National Women's Soccer League.

        View this post on Instagram                   This year has had the highest of highs but along with that comes challenges and sometimes even falling short of my goals I set for myself in 2019. I’m disappointed I won’t be able to compete with the Pride for the remainder of 2019 due to a knee injury I’ve been managing since the World Cup, and that I can’t be there to help my teammates and my club have more success. Thank you fans, friends, teammates, and teams for all your support. I have already started physical therapy in LA and am eager to get back on the field doing what I love. A post shared by Alex Morgan (@alexmorgan13) on Sep 16, 2019 at 3:02pm PDT

"This year has had the highest of highs but along with that comes challenges and sometimes even falling short of my goals I set for myself in 2019," Morgan wrote.

"I'm disappointed I won't be able to compete with the Pride for the remainder of 2019 due to a knee injury I've been managing since the World Cup, and that I can't be there to help my team-mates and my club have more success.

"I have already started physical therapy in LA and am eager to get back on the field doing what I love."

The Pride said in a statement Morgan had "a patella stress reaction in her right knee" stemming from an injury sustained while with the national team.

"It's obviously difficult to lose a player of Alex's calibre due to injury," Pride general manager Erik Ustruck said.

"However, after consulting with the club’s medical team at Orlando Health and U.S. Soccer, we felt it was in the best interest of Alex's long-term health to shut her down for the season to focus on the rehabilitation process and take the appropriate time to recover from the lingering knee injury."

The striker has been nursing the injury since she was substituted during USA's group game against Sweden at the World Cup in June.

She returned to play in the remaining games of the tournament, won by USA as they defended their title, even scoring a goal in a semi-final victory over England.

Morgan, who has played six games with the Pride this season, four before the World Cup and two since, has a history of knee injuries.

She suffered a torn anterior cruciate ligament as a teenager in high school and also underwent keyhole surgery on her right knee in 2015.

Morgan, along with USA team-mate Megan Rapinoe, has been nominated for The Best FIFA Women's Player award, with the winner to be announced at a ceremony in Milan on September 23.