Farmers are cramming millions more chickens into industrial sheds than ever before in order to meet supermarket demand for cheaper food, a Sunday Herald investigation can reveal.

Increasing numbers of birds are being packed into mass meat production facilities because high street retailers want smaller, younger chickens to sell to customers at lower prices. As many as 19 chickens or more are being squeezed into every square metre of floor space, which some experts say causes pain and stress.

Farmers and supermarkets, however, deny that their increasingly intensive production methods are cruel. They point out that animal welfare standards conform to the food industry's assurance scheme and are much better than they used to be.

But, according to Tim Lang, professor of food policy at London's City University and a former government adviser, chickens have the most miserable lives on farms. "Probably no animal farmed intensively has a shorter, more captive or controlled life than the broiler chicken," he said. "A luxury item six decades ago has become routine, tasteless, so-called meat today. And now we witness this new shift to even shorter lives, driven by market changes."

Lang urged people to question why mass-produced chicken has become so tasteless. "If you want to eat chicken, pay more," he suggested. "Eat it more infrequently to compensate for buying better quality."

Controversy has been sparked by a bid from a big farm in Fife to boost its production capacity by almost 50% from 340,000 to 500,000 chickens. In an application to the Scottish Environment Protection Agency, Peacehill Farm on the Firth of Tay says this is because "the supermarkets are requesting lighter birds".

A farm spokesperson said: "The reason for increasing bird places is that due to customer requirements the birds will have a shorter productive life and will be slaughtered at a lighter weight."

The application says this will lead to more chickens in the same space, but promises that the stocking density will not exceed 38 kilogrammes per square metre. If the chickens weigh an average of 2kg each, that is 19 in every square metre.

According to Libby Anderson, policy director at OneKind, an Edinburgh-based animal rights group, this will inevitably cause the birds suffering. The European Union's scientific committee on animal welfare concluded that at more than 30kg per square metre there is a "steep rise in the frequency of serious problems".

Anderson added: "The demand for chicken seems to be limitless and we are concerned to see the drive towards greater intensification of this sort. The more densely they are stocked, the greater the risk of lameness and painful leg problems, hock and foot burns, and stress."

She urged people who buy chicken sandwiches from supermarkets to be aware that the meat comes from birds unable to move around freely. "Current guidance is for stocking densities to be lowered, not raised, when birds are being reared to lower slaughter weights, so we can't see a justification for increasing it here almost to the very upper limit."

Compassion in World Farming (CIWF), a charity that campaigns to improve animal welfare standards, said it was impossible to provide "an acceptable standard of animal welfare" with a stocking density of 38kg per square metre. "It restricts their ability to be chickens," stated CIWF's campaigns director, Dil Peeling. Crowded conditions inhibit the birds' natural motivations to peck, to move and to explore, he said. Heavy breasts, bred for maximum white meat, accentuate leg problems, he said, and some suffered heart failure.

CIWF's director of food business, Dr Tracey Jones, argued it is wrong to try to increase the number of chickens per square metre. "Rearing more, lighter-weight birds in a shed will obviously leave them with less space to move around," she said. "Lighter-weight birds are more active and therefore need more space to move around more freely, rather than less space."

According to Sunday Herald food writer Joanna Blythman, supermarkets want smaller chickens because they can sell more of them at lower prices. "People don't pay a lot of attention to weight," she said. "Lighter chickens seem cheaper even though it's the same price per kilo."

She pointed out that Tesco had previously come under fire for selling small whole chickens for as little as £1.99 in 2008. The supermarket chain was attacked at the time by farmers for short-changing suppliers and by animal welfare groups for encouraging factory farming, though it disputed the allegations. Tesco insisted last week it had not been requesting farmers to produce smaller chickens. But, according to a company spokesman, the chickens it sold ranged from a tiny 0.9kg to 2.7kg "in order to offer customers a choice".

Morrisons, however, accepted there was growing demand for smaller birds as they are cheaper. "People are feeling the pinch because of the recession and that drives retailers to get a wider variety of chickens," said a company spokesman. "It's a cost decision - they want cheaper chickens."

This was backed up by the British Poultry Council, which represents chicken farmers and processors. Supermarkets want smaller birds because of "changing consumer demand", said chief Andrew Large, who added: "The rise in smaller families and one-person households means the larger bird required for a family Sunday roast is less popular in 2014 than before."

He stressed, as did Tesco and Morrisons, that chicken is produced in accordance with the Red Tractor assurance scheme. "Birds raised to Red Tractor standards have access to everything they need, including sufficient light, heat, water and food," Large claimed. But critics point out that these standards allow stocking densities up to 38kg per square metre.

Sainsbury's has adopted the higher standards of Freedom Food, set up by the Royal Society for the Prevention of Cruelty to Animals. It only allows a maximum stocking density of 30kg per square metre.

Farmers' union NFU Scotland said there should be no implications for animal welfare as long as strict guidelines are followed. If the chickens are smaller, there may be more of them but they would still be allowed the same space per kilo, argued the union's animal welfare policy manager, Penny Johnston. "There are codes of recommendations from the UK Government on poultry welfare that producers must conform to. Also, if producers are supplying supermarkets there will also be set guidelines that must be met which have been set by the retailer itself."