The lives of almost 80 million cheap chickens are to be improved in response to growing public concern about their cramped and dirty conditions.

Three chains, Sainsbury's, Waitrose and the Co-op, have promised to transform the welfare of standard birds crammed into sheds and reared unnaturally quickly.

In the single most important measure, Sainsbury's promised to make all its cheap chickens meet RSPCA standards, improving the lives of 72 million birds. Waitrose, the fast-expanding food arm of the John Lewis Partnership, said it aimed to make all of its standard chickens free-range in the long run. It sells four million a year. By March, the Co-op, the supermarket with the most stores, aims to make all its standard birds "slower growing" and give them more space.

The changes are welcomed by the chef Hugh Fearnley-Whittingstall in a Channel 4 special next week in which he chronicles his fraught attempt to convert Axminster in Devon into Britain's first free-range town. The TV presenter encourages the town's kebab shop, butcher, and other traders to sell free-range birds and promotes its benefits to local people. Fearnley-Whittingstall who was ejected from the town's Tesco branch during filming also seeks to change the buying policies of the supermarkets.

He claims representatives of Tesco, Somerfield, Asda and Morrisons fail to meet him or come up with any changes. But, in a draft of the programme, Fearnley-Whittingstall says: "Since my campaign began three of the major supermarket stores have made commitments to improve conditions for the chickens they sell. Sainsbury's have promised to upgrade all their standard birds to the RSPCA Freedom Foods System."

Yesterday, Sainsbury's which said the decision had not been made because of the series could not give a date by which it would achieve the target, but said it would not be before 2010.

However, if it happens, the change would have a major effect on bird welfare because 80 per cent of the 90 million chickens sold every year at Sainsbury's are standard indoor birds.

At Waitrose, Richard Hodgson, buying director, agreed that it was feasible for the store to make all birds free-range but he warned it would not happen in 2008. The store's basic chicken already meets the RSPCA's stocking standard, which sets a maximum stocking density of 30kg per square metre, about 15 birds a metre.

Last year, the Co-op announced all the chickens it sells would be moved to the higher-welfare Elmwood scheme. Although these moves are helpful, 95 per cent of chicken sold in the UK will continue to be reared under intensive systems in the next two years. Only 5 per cent is free-range or organic.

Yesterday, thousands viewed a covertly filmed video of lame chickens in unnatural conditions inside vast sheds at a chicken farm in Herefordshire on the Independent website.