Kirby, a key player for both club and country, has not featured since February and won’t return to action until the issue has been investigated and resolved.
The 28-year-old could miss out on this summer’s delayed home European Championship with England in addition to being unable to help defending Women’s Super League champions Chelsea in their pursuit of a WSL and FA Cup double in the weeks ahead.
Kirby previously spent more than nine months out of football between November 2019 and September 2020 after being diagnosed with the rare condition pericarditis, an illness which causes inflammation to the fluid-filled sac around the heart.
“No chance of her playing at the weekend,” Chelsea boss Emma Hayes said of Kirby ahead of Sunday’s FA Cup semi-final against Arsenal at Meadow Park. “She’s been taking some time in the background, been suffering with a lot of fatigue.
“I don’t have a definitive reason why that is, we don’t know but we’re trying to get to the bottom of it and taking the time we can to try and find the right solutions for Fran.
“This is something that’s been ongoing and unfortunately until we get to the bottom of it Fran won’t be available for selection.”
In a subsequent message posted to fans on social media, Kirby said she was putting her health first as she vowed to return to the pitch as soon as possible.
“I’m sad that I have to write to you all another message like this,” she wrote on Twitter. “With this being an on-going issue throughout my career, it was time to put my health first. Thank you to everyone who has taken the time to message, I’m doing everything to try and come back once again.
“But until then, I will be Chelsea and the Lionesses biggest fan with you all.”