Porter left Leicester at the end of last season. He played his last game for the club against Northampton in the Premiership back in November.
He won his last cap for England against Wales in a warm-up match ahead of the Rugby World Cup last year.
Porter had signed for the Western Force in Super Rugby Pacific but the after-effects of his injury prompted a re-evaluation.
Porter told Rugby-Dump: “I had a particularly long-term injury from November which I struggled to get back from. I spoke to a few doctors and neurologists along the recovery journey and tried to work it out in my head.
“Ultimately, I decided that I'd pushed it as far as I'd like to.
“It was bound to have an impact on my approach to the game, and I only really enjoyed it if I was willing to put my head in the spokes. As soon as I felt like I didn't have the desire to do that, I realised it was probably time to move on while I was still feeling positive about it.
“I finished my contract and, in the context of all that, I just decided not to look for anything else because I felt like the ongoing risk outweighed my desire to continue.”
First selected by Eddie Jones, Porter won six England caps, playing in two Tests on their 2022 tour of Australia. He was also part of Leicester's Premiership title win that same summer.
READ MORE: Tonga's Pat Pellegrini quits Coventry for Super Rugby