O'Connell, 29, a mainstay of the Blades side who achieved two promotions under Chris Wilder and reached the Premier League in 2018-19, suffered a serious knee injury in training in 2020.

Despite attempting a comeback, the Liverpudlian was never able to recover from the problem, with his last professional appearance coming on September 21, 2020.

His Blades deal expired in June and he was left Bramall Lane at the end of the last campaign.

Sheffield United's Jack O'Connell, who has announced his retirement. Picture: Simon Bellis/Sportimage.

Centre-half O'Connell, who was tipped for full England honours in the autumn of 2019, joined United from Brentford in the July 2016 and spent seven years at the club. He also had loan spells in Yorkshire at Rotherham United and York City.

Confirming his retirement on social media, he posted: "This is a very difficult announcement for me to make, although Sheffield United fans will already know that for the past three seasons, I have been fighting to save my professional career.

"Today, I am announcing my retirement from football.

"I have put my heart and soul into this recovery since my knee problems began in June 2020 but this is a battle I cannot win."

"A battle I cannot win" - Sheffield United defender Jack O'Connell announces his retirement from football at 29