Companies behind the breeding stock that supplies annually 860 million broilers for the UK table are bringing in the new breeds to take advantage of the burgeoning market, after years of producers being accused of raising hens in as short a time and as cheaply as possible.

The Royal Society for the Prevention of Cruelty to Animals (RSPCA), which certifies higher welfare standards through its Freedom Food scheme, believes the partial reversal of the faster farming method could lead to a doubling of hens labelled under its scheme, from 5% at the end of 2007 to 10% by the end of this year.

The Guardian reports that increasing numbers of birds reared indoors now live up to two weeks longer than the five to six weeks allowed them in an industry where only one in 20 birds is raised through an outdoor system via free-range and organic flocks.

Typically, consumers pay an extra 60p to £1 for an RSPCA scheme bird. The charity's scheme is the only 'higher welfare' programme to demand that birds do not grow by more than 45g per day. This rule was introduced in 2006 and last February toughened to cover a bird's genetic propensity for growth, following suspicions that some producers were simply feeding fast-growing birds less.

The RSPCA believes the arrival this year of the 'new' chickens 'tailored' to meet these demands will give added impetus to welfare improvements.

Marc Cooper, a farm animal scientist with the RSPCA, said the moves are more important than lower density stocking and improved broiler house lighting. "High growth rate leads to significant health and welfare concerns, including lameness and heart problems. It is such a good issue to tackle yet was one of the hardest to make a move on."

Aviagen, an industrial chicken breeder, says its newest breed of bird is "moving in that direction", and a bird developed by another large breeder, Hubbard, already meets the RSPCA requirements.

"For the biggest global breeders to move in this direction is a real breakthrough," Mr Cooper said.

The move follows years of campaigning by animal welfare groups and criticism of industrial chicken production by the British TV chefs Jamie Oliver and Hugh Fearnley-Whittingstall.

Breeding firms have been looking to France where a significant part of the market adheres to the Label Rouge production standard, which requires poultry to live mostly outside and for at least 81 days. Indoor broiler hens in the UK are never allowed to live that long but given a slower growth rate, coupled with their active nature, they could have lives that theoretically meet the more demanding welfare standards.

The slower growing breeds have been created by mating French birds with animals from the British stock.