An abandoned dog was found with hair so badly matted it had cut off the circulation to one of his feet, causing it to rot off, the RSPCA said.
Benji, a poodle cross, was found dumped at a roadside at Eye near Peterborough on Wednesday.
Vets shaved off 1.2kg of fur and found one of his feet was missing.
It took them two hours to cut all of the hair off while Benji was on a drip and sedated.
The RSPCA said skin had grown over the stump of the missing foot.
"It is likely the foot could have been missing for months, if not longer," an RSPCA spokesman said. "It is thought this was the result of the matting which caused the circulation to stop.
"Without a blood supply his foot effectively died."
Benji, who is thought to be around seven years old, was severely dehydrated and could hardly move when he was found by a group of teenagers who looked after him until the RSPCA arrived.
RSPCA inspector Carrie O'Riordan said: "I was truly horrified when I saw Benji, he didn't even resemble a dog.
"Apart from the matting he smelt extremely bad and his fur was coated in faeces, urine and fleas.
"In all my years as an inspector I have never seen a matted dog as bad as this.
"The person who let him get to a condition like this is truly the lowest of the low, there is just no excuse for it."
Benji was found down a track at the bottom of White Post Road, and is currently at an RSPCA centre near March, Cambridgeshire.
"I would like to thank the young people who found poor Benji," Ms O'Riordan said.
"They stayed with him until I arrived and they wrapped him in their shirts to keep him warm and comfortable. They even played him music to keep him calm.
"I fear he may not have been found had it not been for them and he could have died a slow and painful death."
The vet who treated Benji described the case as the "worst case of neglect" she had ever seen.
The RSPCA said Benji was "extremely nervous and scared" when he was found, but is "starting to come out of himself and is very friendly and sweet".
Anyone who recognises Benji or knows who owns him is asked to call the RSPCA on 0300 123 8018.