They say the natural beef bone, more than a foot long, is making fun of dog attacks on postal workers.
But the manufacturers of the £1.49 treat insist the name is purely tongue-in-cheek and pet owners see the joke with several thousand of the bones sold every week.
A spokesman for the Communication Workers Union said: “Whilst we appreciate the intended humour in the name, over 26,000 postal workers have been attacked and injured by dogs in the last six years.
“Two postmen were nearly killed in attacks in 2007 and 2008, and many others have lost fingers and parts of limbs.
“Reckless and negligent owners who fail to act when their animals attack and injure postal workers or anyone visiting their home, can now face prosecution.”
More than 26,000 posties have been attacked since 2008
The Postman’s Leg bone is sold all over the UK and Europe and is produced by Dugdale Davies Pet Treats, based in Lancashire.