The 46-year-old Irish singer has spoken publicly about his health issues over the years which led to his hiatus from the group in 2024.
According to The Sunday Times, the singer's health complications began in August 2020 after he underwent a laparoscopic sleeve gastrectomy, a type of weight-loss surgery.
The newspaper reported that bariatric surgeon Nick Carter failed to remove enough of his stomach during the operation.
The tissue left behind allegedly caused a leak leading to severe abdominal infections, recurrent sepsis and hospital admission until in 2023 he had to undergo an operation to have his entire stomach removed, known as a gastrectomy.
Feehily's legal team at Leigh Day Solicitors have since filed a medical negligence claim at the High Court against Carter.
Leigh Day partner Suzanne White said: "We are representing Westlife lead singer, Mark Feehily in a medical negligence claim at the High Court against laparoscopic, upper gastro-intestinal and bariatric surgeon Mr Nick Carter.
"Mark has been dedicated to keeping his fans updated on his health during a period that has been extremely challenging for him, but he has faced it all with his characteristic strength and resilience.
"Mark very much appreciates the continued support. In order to respect the ongoing court process, he will not be commenting at this time."
Mr Carter has been contacted for comment. The Sunday Times reported that he denies negligence.
Feehily pulled out of concerts with Westlife in 2022 because of pneumonia and eventually announced he would be stepping back from the band in February 2024 ahead of the group's tour.
Westlife was formed in 1998 and made up of bandmates Feehily, Shane Filan, Kian Egan, Nicky Byrne and Brian McFadden.
They disbanded after 14 years in 2012, but reunited in 2018 without McFadden - who left the band in 2004.
The remaining trio are currently on an anniversary world tour to mark 25 years since the band first toured together, including a 13-night record-breaking residency at 3Arena in Dublin.
The residency will wrap up on Saturday night before the band play shows in Glasgow, Manchester, London and Cardiff.
Westlife have sold more than 55 million records and have had 14 UK number one singles and eight number one albums.
The group is best known for the hit songs Flying Without Wings, You Raise Me Up and If I Let You Go.
