Mischief was posed next to the weight in butter which he lost

Mischief, a fat feline from Stevenage, lost 38% of his bodyweight over 40 weeks to be handed the title at an event in central London on Sunday.

Owner Sharon Harding said she did not realise how unhealthy the six-year-old had been until she took him to the vet.

Mischief has since lost 5kg, now weighing in at a trim 6.3kg.

Ms Harding, 35, who lives with retired parents Richard and Beryl and four other cats said: "He was the only fat one. The other ones were normal. He must have been eating all the food.

"Mischief couldn't fit through the cat flap, he couldn't jump onto the bed or even wash himself. We now have a tree-climbing cat. I couldn't be happier or prouder.

"He still borders on the lazy, but now he moves more than he did."

A vet was horrified at how fat Mischief was

The other winner was Max, a three-year-old collie-Labrador who once weighed in at a gigantic 55kgs.

He lost 40% of his bodyweight in the space of 24 weeks. He slimmed down to a much more manageable 33kgs.

Owner Maggs Gray, who lives on a small farm in the village of Milnathort in Tayside, bought him from an animal shelter.

She said: "He was so fat. He needed an energetic owner and a new home.

"He was grossly overweight and he couldn't even do much exercise. He would just waddle about."

The prize, in its 12th year, was sponsored by Hill's Pet Nutrition.

The winners were given a year's supply of pet food, �500 worth of pet accessories and �2,000 in holiday vouchers.