During a family holiday on the Scottish island of Jura, the Prime Minister said a scan showed he had a protruding disc.
Mr Cameron, who enjoys deer stalking, said his back pain meant he avoided the hunting activity, which requires crawling on the ground.
His comments came during a pre-recorded interview with the BBC. Mr Cameron said about the whisky-producing island: "It's a great place to unwind. I like the great outdoors. I like an active holiday.
The island is home to thousands of red deer and according to the Prime Minister deer stalking is "probably one of the most defendable" field sports. He also denied giving up hunting because of opposition to the sport by some people.
"I find now I'd rather go for a walk and this year I've got a phenomenally bad back so I really don't think I'll be able to crawl," he said.
More top news
Dry and sunny tomorrow and feeling warm.
Mild tonight with warm sunny spells and light winds tomorrow.
A section of the M3 has been closed and bomb disposal units are at the scene after reports of "potential hazardous material in the road".