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."
Gilbert has released a new line of rugby cleats. The Gilbert Virtuo 8S is part of the exciting new product. Check it out.
The entire All Blacks apparel line has been updated for 2013/14. Check out the New Zealand All Blacks polo.
The Nike Tiempo is a solid rugby cleat and one of few styles still made from full-grain natural leather.
The Gilbert Blitz 8S rugby cleat is a great cleat at a great price of $69.99. Get a new pair of cleats today.
A cool looking all black rugby cleat with the high performance adidas is known for. Get in the Gear!
Wear the crest of the British and Irish Lions on your t-shirt. A great look for the summer.
The All Blacks Performance t-shirt is black with hints of blue from the training jersey. Very Cool.
The New Zealand All Blacks training jersey for 2013/14. Get in the Gear!
The USA Rugby Pro Alternate rugby jersey is perfect for any fan of the Eagles. Get yours to wear during the summer Test matches.
The NEW All Blacks 2013/14 jersey has arrived at World Rugby Shop. Dare to wear the colors of the All Blacks.