Long, 34, was set to leave Northampton this summer and join their Premiership rivals Gloucester, but those hopes have been dashed due to the effects of a degenerative neck injury.
Long won two Test caps for England, making his debut in Clive Woodward's first game as head coach against Australia at Twickenham in 1997.
"I have achieved everything I wanted to achieve out of rugby," said Long, who also had spells with Bath, Munster, Rotherham and Newcastle.
"I've had 200 games of Premiership rugby and played for some great clubs who have all had magnificent supporters, and I am at peace with the fact I'm having to retire.
"That said, I had signed for Gloucester so I wanted my career to continue, but I'd been carrying an injury which was causing me a few problems towards the end of last season and the rehabilitation time from an operation would have meant that I would have been on the sidelines well into next season too."
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