Animal welfare campaigners are stepping up their battle against a toy which they say is cruel to fish.

The novelty products are being sold in the run up to Christmas as executive toys and are designed to house two Siamese Fighting Fish, or Betta Splendens, in a small plastic fish tank.

The idea of the toys is to place two of the South East Asian fish at either side of a plastic dividing wall in the tank. The product's promoters say when the fish are placed close to each other, the fighting fish become stimulated to intensify their colours, spread their fins and flare their gills, creating an eye-catching display.

But inspectors from the Royal Society for the Prevention of Cruelty to Animals in Brighton claim the creatures will be placed under unnessary stress if they are forced to live in the plastic tanks and are urging people not to buy them.

They also claim the conditions in which the fish would be kept are unsuitable and say the creatures need much more care than the manufacturers suggest in the guidelines printed on the packaging.

RSPCA inspector Barbara Kvalheim said: "We can't take any action because these toys are not illegal but we are advising people not to buy them.

"The fish would be constantly under the stress of threat because when they are in their natural habitat and they come across each other, they flare their gills, fight, but then back away and hide.

"In these toys, they have nowhere to go and can constantly see each other which places them under a lot of stress."

The RSPCA fears people will snap up the toys as unusual presents, without thinking of the suffering that may be caused to the fish and it is holding talks with the manufacturers.

There are a range of makes of this type of fish tank toy on the market and the RSPCA says it has discovered them on sale in a number of shops and supermarkets.

The society's regional manager Richard McCrow said: "Fish require a stable environment to ensure their health and welfare. This stability must include water temperature and the chemical quality of the water, as well as adequate oxygenation."

But Mr McCrow said that it is impossible to provide a suitable environment in the small volume of water used in the novelty toys.

Manufacturers claim Betta fish are accustomed to living in small pockets of water and that they are labyrinth fish, which means they breathe from the surface. They say this helps to eliminate the need for a complicated filtration system, making the toys a low-maintenance tank.