Owners of fat dogs or cats could face prosecution under the Animal Welfare Act which comes into force tomorrow.
The Act, the biggest overhaul of animal welfare legislation for a century, creates a new offence of failing in the duty of care towards a captive animal.
Pet owners can for the first time be guilty of an offence before an act of cruelty has been committed, for example by overfeeding their pet.
The Act says a person responsible for an animal must provide it with a suitable diet, intake of water, environment and housing and ensure it can behave normally and is without pain or disease.
Behaving normally means that sociable animals, such as dogs and rabbits, must be provided with companionship, either of their own species or humans.
Maximum penalties for breach of the new rules include a ban on owning animals, fines of up to £20,000 or up to 51 weeks in prison. The Act raises from 12 to 16 the minimum age for buying a pet and bans pets as prizes for under-16s.
Docking of dogs' tails for cosmetic reasons is banned with exemptions for "working" dogs used by the police, Armed Forces, search and rescue and gun dogs.
A vet in England may dock a dog's tail as long as it is no more than five days old and its owner has provided the following evidence: the dam of the dog (so the type may be ascertained), a completed statement by the owner and a shotgun certificate, police identification or other evidence that the dog will be used as a working dog.
The law will be different in England, Scotland and Northern Ireland.
In Wales the word breed is used rather than type, which appears to mean that if a dog is not of a pedigree listed in the regulations its tail may not be docked. In Scotland there is a separate Act which bans docking for any reason. In Northern Ireland tail docking is legal.
The RSPCA said that its inspectors would give written advice to pet owners in the first instance in the case of an overweight dog. A date for a return visit would then be given. In the event of obvious neglect a warning notice would be issued or the owner prosecuted.
In London's Hyde Park yesterday some dog owners were concerned about the new powers.
Vivien Battarbee, 61, a designer from South Kensington, owner of Missy, a Parson Jack Russell terrier, said: "I'd be horrified if I had a knock on the door from a policeman if my dog was overweight. Jailing would be going too far, but maybe it would be good to remind some people how to look after a dog."
Kenneth Stern, 78, who was walking Worcester, a Dalmatian, said: "Convicting people is a bit over the top, but some people simply do not realise how to take care of their dogs."
The Pet Health Council says:
Run your hands along the dog's sides to see if you can feel its ribs when you apply gentle pressure. Then check from above whether you can see a waist - a thinning from where the ribs end to where the hips begin. Finally, run your hand gently over its back to check if you can feel hip bones. If your pet has all of the above it probably will not be obese.
If your pet is slow to get up and has trouble moving around there may be a problem.
Specil Thanks to The Telegraph