The dog was found in a lane near the Leigh Delamere services
A terrier was found with string tied so tightly around his neck that it had cut into his flesh and muscle.
The dog was found in a lane near the Leigh Delamere services on the M4 near Bristol on Tuesday, an RSPCA spokesman said.
The animal has now been treated and taken to Bath Cats and Dogs Home.
The RSPCA's Ian Burns said: "I've done this job for years but I was shocked to see how bad this injury was and how long the dog must have been in pain."
He added: "He was probably tied up or tethered with this piece of string cutting into his flesh and the more he tried to pull away the more painful it would have been."