Sales of factory-farmed chickens have slumped since a high-profile campaign raised awareness of the cruelty at the heart of the poultry industry and implored consumers to pay more to improve the animals' welfare.

In a victory for campaigners who have fought to expose the short and brutal lives of broiler birds, shoppers have bought millions more free-range and organic birds while leaving mass-produced chickens on the shelves.

Sales of free-range poultry shot up by 35 per cent last month compared with January 2007, while sales of standard indoor birds fell by 7 per cent, according to a survey of 25,000 shoppers by the market research company TNS.

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Supermarkets have been stripped of free-range birds, prompting complaints from frustrated shoppers keen to embrace the movement away from intensive farming.

The rise in sales would have been even higher if poultry producers had been able to keep up with demand. Many suppliers in the £2bn-a-year poultry industry are now expected to convert cramped chicken sheds into more spacious accommodation.

Tesco, the country's biggest retailer, has doubled its order for higher-welfare chickens while Sainsbury's has been flabbergasted by the "unprecedented" spurt in demand and forced to import free-range birds from France.

In the weeks after the chefs Hugh Fearnley-Whittingstall and Jamie Oliver launched a high-profile campaign on Channel 4, supermarkets had stated that sales of "standard" chickens had held up, and even increased.

But the new national sales data suggests that shoppers' priorities have shifted dramatically. If the TNS data was extrapolated to the rest of the UK, it suggests sales of factory-farmed chickens dipped by 10 million, while shoppers bought 4.4 million more free-range chickens. Overall, chicken sales were down by 4.8 per cent, perhaps because many people, when faced with an absence of free-range chicken, simply bought no chicken.

The campaign against mass-produced poultry, of which a quarter have difficulty walking as a result of wading around in their own waste, is to be intensified. Fearnley-Whittingstall intends to produce a new television show on chickens later this year, updating viewers on the campaign and urging more people to join what he hopes will turn into a free-range revolution. "We are going to keep the pressure up and we are going to do everything we can to make sure that this is not a flash in the pan," he said.

During his Hugh's Chicken Run shows, residents of the Devon town of Axminster were invited to see free-range and intensive systems running alongside each other in a shed; many left in tears. According to separate polling by ACNielsen, half of the four million viewers who saw the shows said they would buy better chicken.

The cruelty inflicted on broiler birds was also exposed in secret footage from a farm, reported last month in The Independent. Earlier this month – to the disgust of the National Farmers' Union and animal welfare groups – Tesco announced a week-long offer of a £1.99 chicken. The move is believed by welfare campaigners to have been an attempt to shift unsold standard birds.

"If the growing consumer demand for free-range, organic and higher-welfare chicken continues, availability in store could certainly become an important barrier to consumer choice, at least in the short term," said Maria Carrol, ACNielsen's consumer insight manager.

Compassion in World Farming, a campaign group which shot undercover footage inside a chicken shed in Herefordshire, was jubilant. "It seems to me that there is a swath of people who have been moved by the programmes and it seems to be a lasting move, a definite move away from standard to free-range," said its food policy officer Rowen West-Henzell. "That's great. But what we need to do is to work with the people who still buy standard and we are 100 per cent committed to giving consumers the facts about poultry production and letting them make their own minds up. With the programme they were exposed to that reality."

About 800 million chickens are bred in the UK every year. About 92 per cent of them are still of the "standard" variety, despite the increase this year.

"I am thrilled but I am still a little bit cautious," said Fearnley-Whittingstall. "I am delighted we have helped create this change and I am delighted that, two months after the show, there appears to be no letting up.

"I just hope the British retailers and the industry are talking to each other, making sure that new free-range farms are built and new RSPCA Freedom Food farms are built to cater for a growing demand for high welfare chicken."

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