"This is food as a basic necessity. They are coming because they're hungry." At Perfect Fried Chicken in Finsbury Park, north London, 26-year-old owner and manager Mohammed Miah talks to me in snatched moments amid an unending stream of orders. "We've got regular customers who eat it twice a day. I'm not judging them. Everyone's got to eat. The government is always criticising fast-food companies but we are not pulling people off the streets."

Nor do they need to be. Fried chicken is the fastest growing of all fast foods. In 2008, according to a Mintel report, fried chicken represented 4% of the country's eating-out market, a sector estimated to be worth £15bn-£20bn. Its market share looks likely only to increase; fried chicken retailers saw sales grow by 36% from 2003 to 2008, compared with just 22% in the fast-food sector as a whole. Some councils are now taking measures to halt the influx of cheap takeaways.

Earlier this month, councillors in Oldham in Greater Manchester green-lit a plan, dubbed by local papers a "fat tax", to impose a £1,000 fine on new takeaways opening in the area. The plan's proposer, Labour councillor Steven Bashforth, says it wasn't about obesity so much as the character of the town. "One of the things that actually triggered this was that KFC was given planning permission to open in a district centre that already had 16 takeaways. We literally have streets that consist of nothing but kebab shops, chippies, curry houses, pizza, chicken and burger outlets. To be quite frank, it's a mess."

The situation in Finsbury Park is similar. Competition, Miah says, is fierce. "In fried chicken right now, the industry is booming, especially in London. In the last six months we have had four open up in this area alone." By my count there are now six other fried chicken retailers within a minute's walk of his own.

He is not as worried for his business as you might expect. Despite an ever-more crowded marketplace, Miah's store is still regularly packed. "You've got the lunchtime rush, you've got the school rush and you've got the evening rush. One moment it's quiet and the next moment the whole shop is full." As if on cue, a dozen young men step in from the cold, and the counter buzzes with hungry anticipation as the serving staff bellow their orders back into the kitchen.

One customer is especially keen to get his money's worth. As Miah's staff fill him a box of chips and chicken, he asks for mayonnaise. "More," he says, "more, more." When he's handed the box, the food now nearly hidden beneath thick white dollops, he reaches for the ketchup and fills in the gaps. Then he reaches for the barbecue sauce. And then the mustard. "I haven't eaten all day," he says, pre-empting the judgment of no one in particular.

The industry has come a long way since Colonel Harland Sanders set up the now-ubiquitous (and abbreviated) Kentucky Fried Chicken brand in the early 1950s. A decade later there were more than 400 stores across America and Canada. In 1965, the secret recipe found its way to the UK, and the first shop was opened in Fishergate, Preston. It was one of the first fast-food restaurants to come to the UK from the United States. Now the franchise here boasts more than 800 outlets, with another 38 due to open this year.

KFC is the undisputed market leader. Even at the height of the recession, KFC's sales continued to grow, and its success has spawned a wealth of imitators, from Tennessee Fried Chicken to Texas, Dixie to Chicago, Perfect to Golden. They owe more than just the formulaic name to their market leader. Many of the smaller franchises' founders and managers took their first steps in the business at KFC. One such graduate is 49-year-old David, the ever-smiling proprietor of Hollywood Fried Chicken in Earl's Court, London. A veteran of the industry, David started working at KFC aged just 17.

"It wasn't that big back then," he tells me. "The shop was just like a post office. There were no machines to show your produce to the customer, and there were no other items on the menu, just chicken." David is keen to emphasise how much the industry has changed, and how much he in particular has taken steps to make his food, if not exactly healthy, at least healthier. In a quiet moment between serving customers, he ushers me behind the counter and lifts a silver display tray, revealing a small pond of boiling water underneath. "When the chicken comes out it's covered in fat, so we let it steam," he explains. "To you it's fresh, but I know it's unhealthy."

I ask him what he makes of the view that fast food has led to obesity; whether he feels the industry is at all to blame for the fact that a quarter of all British adults are now overweight. "At the end of the day, if they want to be obese they can be obese. You don't become obese because of fried chicken. I know it's a hazard to your health but it depends how you discipline yourself."

Fat is not the only substance in which fried chicken is often hazardously rich. One of the primary causes of high blood pressure is a key ingredient in the moreish taste of fried chicken and fries: salt. High blood pressure is one of the primary causes of strokes, heart attacks and heart failure, which account for around half of all deaths in the UK.

Katherine Jenner, a public health nutritionist, is campaign director at Consensus Action on Salt & Health (Cash). Thanks, in large part, to targets campaigned for by Cash, the UK now leads the world in salt reduction, and the salt content of supermarket foods have fallen by around a third. The same cannot be said for our fast-food outlets.

"The salt targets deal with retailers but don't cover the out-of-home sector," explains Jenner. "Particularly among young people and lower social groups, things like fried chicken have become the mainstay of their diets. It's a gram of salt in a KFC chicken wing. You can be assured that the ones at the local outlets will be a lot more than that. Sainsbury's is selling a chicken wing with 0.17g of salt per wing. That's less than a fifth. So there's absolutely no need for them to have this much salt in them."

On both salt and fat, Jenner is keen to emphasise, KFC has been admirably co-operative. It was one of the first companies to reduce drastically its use of trans fats, and stopped salting its fries in 2005 as part of a wider salt-reduction programme. It is the small businesses that are hardest to convince to change.

At Hollywood Fried Chicken, David has already taken measures to address the problem. "I have made the menu much healthier," he says. "I cut down on salt. The customers have a choice. They can add salt if they want." He says he welcomes efforts to make people eat healthier. "If the government influences people to eat healthy we all have to respond to those changes. It all depends on disciplining the people, not disciplining the businesses. We are here for the consumer. The consumer has a choice. People choose fried chicken because the prices are a lot lower than any other food."

Not everyone is as confident in the power of consumer choice. Professor Graham MacGregor, chairman of Cash, takes issue with the view that businesses should wait for their consumers to change. "Obviously, in an ideal world you would change consumer choices, but it's almost impossible with salt. For the more deprived people in this country there's no way they have any choice when they buy the cheapest food, there's no way they're going to look at the salt level."

At Chicken Express in Finsbury Park, owner and manager Balasingham Pathmaseelan puts it plainly: "Fried chicken won't work in Chelsea, Kensington or Hampstead, or anywhere like that. It's only places for the lower middle class or working class. That's the only place you can do it."

No surprise that it is in the most deprived areas that fast-food outlets, and fried chicken places in particular, seem to thrive. Since 1998, the average income of the poorest households has risen by 17.5%. In the same time, food prices rose by 25%. In the borough of Tower Hamlets, home of the capital's most deprived children, for every school there are 42 fast-food outlets. In inner-city London generally, the figure is 37. In the country at large, on average just 25. Tower Hamlets is what nutritionists call an obesogenic environment. According to one report, some children in the borough eat 16 takeaways a week.

Most, if not all, of the capital's fried-chicken hotspots are to be found in low-income areas. On Whitechapel Road, familiar to most as the cheapest street on the Monopoly board, you can taste the chicken in the air. Here, as in Finsbury Park, there are half a dozen or more fried chicken shops just a stone's throw from one another. Both are low-income areas with large Muslim populations; every one of the places I visit has been certified Halal. In City Fried Chicken in Whitechapel, the prayer call is broadcast to diners from a speaker just beside the counter.

It is here that I meet 20-year-old student Mizan, sitting with a plate of piled chicken wings. No drink, no fries, just 10 small folds of battered brown meat and bone. "I started eating fried chicken when I was one year old," he tells me. "I'm not too bothered about its healthiness. I think it's not too bad." On the table opposite, 21-year-old Hassan is picking over a three-piece meal. Does he worry about his health? "I do worry about how healthy it is – mostly salmonella." I ask him how often he comes here. "I would say I eat it a lot. Probably daily. It's, like, addictive or something."

He's not wrong. "Salt is a very addictive substance," explains Jenner. "The more you have of it, the more your taste buds adapt to it and the more you crave it. Particularly for young children it's vitally important that they don't get used to having too much salt."

Back at Chicken Express, Pathmaseelan is well aware that his regular customers are doing their health few favours. "We have people who eat here every day," he says. "I do try to warn them, but to survive you have to sell it." The majority of his customers this afternoon are young black and Muslim men. The school rush interrupts this pattern: for an hour or so it is mostly children, some in packs with friends, others here with their mothers. The popularity of fried chicken among these schoolchildren is staggering. Further down the road, crowds of two or three dozen gather outside the chicken shops, clutching boxes and greasy legs and thighs, and littering the pavement with tiny, discarded bones.

I sit down with one of the younger customers to get his perspective. Why does he eat it? "Because it's chicken and it tastes good and it fills you up," explains 20-year-old student Akbar, here at Chicken Express for lunch with a friend. "I eat it two or three times a week now." I ask him if he worries about how healthy it is. "As you can see," he says, clutching his rather impressive belly, "I don't really think about that."