As a final precaution, before being cleared for surgery, patients waiting for weight-loss operations in Chester are referred to a bariatric psychologist, Dr Denise Ratcliffe, who assesses whether they understand their eating behaviours and whether they are strong enough to change them.

The meetings can be very emotional, as Ratcliffe digs gently away to disclose the causes of the problem.

"How does the emotional overeating happen?" she asks a morbidly obese 45-year-old woman who has gained weight over the past 20 years, mainly through twice-weekly episodes of binge-eating. If the bingeing continues, fitting a gastric band will not work.

"It's heavily linked to my depression," the woman explains, and describes an unhappy marriage and her husband's mental-health problems. "I don't tend to eat when I'm happy, but when I'm upset, I tend to go for stodgy foods. It gives me a dopamine feeling, a pleasure that is missing elsewhere," she says. "It's a numbing, anaesthetising feeling, the feeling you have after Sunday lunch, that you don't want to move."

Ratcliffe works for the private company Phoenix Health, which specialises in bariatric surgery and is contracted by the NHS to perform such surgery in the north-west of England, from Cumbria to Cheshire. The company expects to perform about 900 bariatric operations this year, 50% of them gastric bypasses, 20% of them gastric bands; three-quarters of these operations will be performed for the NHS. Across England there has been a sixfold increase in the number of bypass operations performed in the last five years, with 5,400 patients undergoing surgery in 2011-12.

The obesity clinic's director, Prof David Kerrigan, knows how frequently overeating is linked to psychological difficulties, through years of experience. These issues need to be identified and understood before surgery takes place, if the operation is to work effectively.

"You need to treat the patient from head to stomach, not just the stomach," Kerrigan says. A study conducted by his clinic on 100 female patients a few years ago revealed 50% had experienced some form of physical or sexual abuse. He feels ambivalent about talking about the findings: "There is probably enough stigma associated with obese women, without this," he says. But he knows that without a clear understanding of the issues the problematic eating will not stop after the operation.

Understanding the psychological problems patients may be facing is not an alternative to surgery. "If your television is broken, you don't necessarily want to understand why it is broken; you just want to get it fixed," he says. Nevertheless, staff need to be alerted to the presence of stress, to avoid operating on someone who is in the grips of a crisis and to avert possible post-operative problems – the risk that binge-eating could continue, the risk that ongoing unhappiness could undermine the patient's commitment to changing their diet.

Kerrigan's first patient of the morning has a problematic gastric band. She attributes her overeating to a difficult childhood. "There were a lot of family problems. If I'm happy I'll have something to eat. If I'm sad, I'll have something to eat. Happy, sad, celebration, whatever, I'll open the fridge," she says.

"There were big family problems; their mum left them as babies," says her grandmother, who has been at the consultation.

When the session ends Kerrigan says: "That is probably true of most of our patients. There are issues of using food as a crutch and patients often have a valid reason for their unhappiness. It may start from a psychological issue but after a while the signs that tell you whether you are hungry or not are blunted, after a point their physiology changes, their appetite control is distorted, so they lose their sense of satiety, and that pushes their weight to the high levels that we are seeing here."

The patients who qualify for surgery on the NHS need to have become dangerously overweight, with a body mass index of 40 or over.

His next patient says she was thin as a teenager, but started eating more in her late 20s because of an unhappy relationship with the father of her children. Her GP has advised her to lose weight urgently because her excess weight is causing serious damage to her back and knees. "I've had horrible relationships. I've had a horrible life … stuck in something I couldn't get out of, that's the problem. When you are stuck in something like that you've got to have something, haven't you? It's better than drinking, isn't it? Or smoking."

Kerrigan asks if she is happier now. "I'm on my own," she says. She is optimistic that surgery might improve things.

He believes surgery can be transformative. "We are not just getting rid of their weight and their diabetes, but their confidence also improves, they begin to re-engage with their families, they blossom again," he says.

He is despondent that many GPs struggle to know how to help severely obese patients. "From a medical perspective, they are the lepers. They are 'heart-sink' patients for GPs. They have psychological baggage and complex medical problems," he says. "The standard approach is to propose diet and exercise rather than surgery, which is still seen as barbaric and unnecessary by large numbers of doctors. But by the time you have a BMI of 50 exercise is physically impossible and patients are frustrated with dieticians who tell them to embark on another diet."

In the neighbouring consulting room Ratcliffe asks the 45-year-old woman how her weight has affected her life. "How hasn't it? It's affected my relationships, my career." She had a "hypercritical father" who would "comment on any hint of chubbiness, cruelly". "It made me feel wretched," she says. She had a mother who made her clear her plate at every meal. In adult life she has felt stigmatised for being overweight.

"There is a lot of 'fatism' in jobs. It is one of the few discriminations which is still allowable. I've had a lot of interviews where I've got there and they've looked me up and down and sneered. I feel angry about the discrimination, about other people's attitudes. I have a close friend in a wheelchair and she suffers far less discrimination than me. But overeating is self-inflicted so it's a different situation," she says.

"It's an addiction, but unlike other addictions, overeating is not something you can just give up. You still have to keep eating."

She is reading a book entitled 50 Ways to Soothe Yourself Without Food, which is helping her to stop comfort eating. She is confident she will be able to control herself after surgery.

The dietician who is sitting in offers practical advice ahead of the operation, suggests the woman might like to consider using a pastry fork so she can transport less food to her mouth for each bite; she also wonders if it might help if she tried transferring her fork from her right hand to her left hand, to further slow the process of eating.

The dietician who is sitting in suggests the woman might like to consider using a pastry fork so she can transport less food to her mouth for each bite; she also wonders if it might help if she tried transferring her fork from her right hand to her left, to slow down the process of eating.

Ratcliffe flicks through the case notes of her next patient trying to understand what caused a 55-year-old taxi driver to become morbidly obese. "What's happening with this guy? He's got a whopping great file," she says, reading of his severe medical problems as he struggles to lose some of his 25 stone. He is off work because the diabetes he has had for 14 years has affected his peripheral vision, and one of his legs. When he comes in he looks overweight in a sturdy way, as if he has packed a portable television into his stomach.

Ratcliffe wants to understand why he began to overeat so dangerously, to see whether there are any psychological factors that might affect how well he responds to surgery.

Part of the problem was the hours he worked, through the nights, ferrying people to and from the airport, which meant it was difficult to schedule regular meals, so he would snack on Mars Bars and stop at the airport cafes for cooked breakfast whenever there was a pause – sometimes several in a morning.

But the worst period came a few years ago, when his brother, also overweight, died of diabetes: "Lost his foot, lost his leg, lost his life," the man says; his father died shortly afterwards and his son became seriously ill. "We went through everything that year. The likes of dieting went through the window. We were in a daze. I look back at it now, and it's a total blur."

"How do your emotions affect your eating?" Ratcliffe asks. "If I was under pressure I would eat … I am a bottomless pit, I can eat and eat and never feel full," he says.

Unexpectedly, taking time off work has helped him to focus on changing his life. "When my eye started to go, it frightened the living daylights out of me. I would hate to be blind," he says. He hates worrying about whether cafes will have fixed tables so that it is impossible for him to squeeze his body in. He hates not being able to play with his grandchildren, and the way people look at him. "In my job you are picking people up who are drunk all the time. They say 'You fat …' There is a stigma attached to the weight."

He has embarked on a new diet, made easier now he is no longer working, and has started eating more fish and plain meat instead of sausages and burgers, and has lost more than four stone.

Some of Kerrigan's patients are impatient with the focus on the psychological links to their weight gain.

In the high dependency unit of the Countess of Chester hospital a 17-stone, 56-year-old woman is recovering from a gastric bypass. She booked in to have the procedure done privately, keeping the operation a secret from everyone apart from her husband. "Everyone thinks I'm on holiday," she says. She cannot see any psychological reason why she has battled with her weight since she was a teenager. "I eat a lot," she says. "I think it's just habit now."