Bilbo, Britain's only lifeguard dog, who patrols Sennen in west Cornwall will no longer be seen on duty.
The RNLI has taken over beach safety at Sennen from Penwith District Council this year.
And it says the 14-stone Newfoundland will be unable to patrol as a lifeguard because of a dog ban on the beach.
Bilbo's owner said the decision to keep a lifesaving asset off the beach was a "scandal", but Penwith council is backing the RNLI.
"He is the biggest asset for beach safety and for the RNLI to disown him is a scandal"
But his owner, Steve "Jamo" Jamieson, 53, has been told by the RNLI that a concession which got Bilbo round a beach ban, was no longer acceptable for safety reasons.
Bilbo had been allowed to carry out his life guarding duties in the past because he had been carried on an all terrain bike (ATB) when he was not at the lifeguard hut or in the sea.
Now that the RNLI has taken over life guarding duties, it is not allowing him on their ATBs.
Rebecca Kirk, chief environmental officer at the council, told BBC News: "If he is on the beach it is against the law.
"It puts us in a really difficult position, but we have to be consistent."
Bilbo is not classed as a working dog, so if he goes on the beach, owner Jamo faces a £75 fine.
Jamo said: "I was dumbstruck when I was told.
"He is the biggest asset for beach safety and for the RNLI to disown him is a scandal."
Steve Instance, the RNLI's lifeguard inspector the South West said: "We have been as flexible as possible.
"But ATBs are for one person only and if a dog is 14 stone he is a passenger and that is simply not safe.
"We are not simply sticking to the rules for the sake of it. There are a lot of accidents on ATBs."
He added: "Bilbo is a fantastic asset and we have told Jamo he can use him for PR work and safety education in schools.
"But it is not suitable for the RNLI to have him as an operational lifeguard.
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