Was their mother's undisguised favouritism for the son she called a musical genius the real reason Karen Carpenter succumbed to the anorexia that claimed her life?

One evening in 1981, the popular BBC1 programme Nationwide broadcast an interview with one of the most loved musical parterships of the time: The Carpenters, a squeaky-clean duo, comprising the velvet-voiced singer Karen and her brother Richard.

They were interviewed by Sue Lawley, and fans no doubt expected to hear about new record releases and concerts that the American pair had planned. Instead, viewers saw a painfully thin, gaunt and fragile-looking Karen sitting on a sofa – her huge, brown eyes staring, hauntingly, from her drawn face.

Lawley asked the question outright: was the reason Karen hadn't been recording or going on tour recently because she was 'suffering from the slimming disease called anorexia nervosa'?

Forty years ago, it was a condition that most people hadn't heard of, and few understood.

Karen, then 31, was instantly defensive. 'No, I was just pooped... tired out,' she protested.

Her brother, sitting by her side, hastily stepped in. He called a halt to filming, claiming that Karen's eating issues were in the past. When the interview re-started, there was no mention of Karen's weight or her gaunt appearance.

Sixteen months later, Karen was dead. Her death, an inquest later recorded, was due to heart failure as a consquence of anorexia.

More than four decades on, a new book and documentary are re-evaluating the life and legacy of one of the world's best-known singers, who became the first celebrity casualty of an eating disorder and whose death put a face on anorexia, triggering widespread attention and research, for which many young people can be grateful today.

That awkward BBC encounter is re-shown in the documentary film, Karen Carpenter: Starving For Perfection, which reveals fresh insights into the singer's life and legacy. The recently published biography, Lead Sister: The Story Of Karen Carpenter by British author Lucy O'Brien, also re-examines Karen's battle with anorexia and body dysmorphia, which saw her consume up to 90 laxatives at a time and her weight plunge to less than five-and-a-half stones.

So what drove this clever, talented and beautiful young woman to starve herself to death?

Contrary to popular belief that fame and public scrutiny were to blame, those close to the Carpenter family claim the root of Karen's problems was an overbearing, perfectionist mother who clearly favoured her 'musical genius' brother, Richard.

Speaking to The Mail on Sunday, Randy Schmidt, a writer and producer on the documentary, says: 'One of Karen's closest friends told me there was a hole in Karen's heart where the love of a mother should have been and it couldn't be filled with the love of friends or even by millions of adoring fans around the world, and she ended up almost wanting to disappear – to make herself smaller and smaller.

'I think her family did love her in their own way, but Karen needed something else and it's unfortunate that their love for one another was so misaligned.'

Born in 1950 to middle-class parents in Connecticut, Karen was three years younger than her brother Richard, a piano prodigy who started arranging music at 12. Their mother, Agnes, often boasted that her son was a musical genius, and Karen idolised him.

In 1963 the family moved to California to help the teenage Richard's burgeoning musical career. There, Karen found her first musical love, playing drums in her high school's marching band. Karen was a typical, chubby teenager and Richard teased her, calling her 'fatso'. Their mother described Karen as 'hefty around the butt' and when she was 17 took her daughter – then a healthy 10st and 5ft 4in – to see a doctor about her weight.

She was put on the Stillman water diet, a high-protein, low-carbohydrate diet popular in the 1960s, which involved vitamin supplements and drinking eight glasses of water a day.

After six months, Karen had lost 25lb, but her relationship with food, and her self-image, was irrevocably damaged.

'That was the start of Karen counting calories daily and exercising. She was determined to stick to it,' says Lucy O'Brien. 'Food began to be seen as the enemy and I think that's what was bedding down in Karen's psyche.'

Driving this obsession, explains Randy Schmidt, was Karen and Richard's perfectionist mother. 'Karen joked that the floor of their garage was so clean you could eat off it. Their mother scrubbed the window fittings on their house with a toothbrush and when she noticed her neighbours' windows were dirty, she'd go over and clean them as well.

'Karen and Richard learned from a young age to try for perfection in everything. It's not a bad thing if regulated. But as people in the documentary say, perfection is impossible, and you continually disappoint yourself when that's the goal.'

The brother and sister signed with A&M Records and their first hit single released in 1969 was a cover of The Beatles' Ticket To Ride. After realising the true worth of her contralto voice – described by some as like that of an angel – the label started to wean Karen away from playing the drums, her greatest love, and – poignantly – an instrument she could hide behind.

By the early 1970s, she was front of stage, alone and with a microphone, and millions of eyes on her.

With timeless classics such as Rainy Days And Mondays and Yesterday Once More, they sold 100 million records worldwide.

In 1973, when she and Richard appeared on The Bob Hope Special, Karen saw pictures of herself in her stage gold trouser suit. She had a barely noticeable tummy but told her brother she looked heavy and intended to do something about it.

'Karen was always worried about her weight but when I first met her, she wasn't anorexic,' remembers Maria Luisa Galeazzi, who worked as Karen's personal assistant, hair and make-up artist. She remembers regularly going out to dinner with Karen and Richard, and Karen complaining, 'You two can eat anything and not put on a pound.'

'But she was eating normally. She loved to cook and at Christmas we'd spend a day making cookies.'

Her relationship with her mother remained anything but normal, however. While her father was a quiet man who stayed in the background, 'her mother ruled the roost and Karen could not please her mum. Richard wasn't picked on as much as Karen.'

Galeazzi and Richard had a romance that was cut short when Karen told her assistant that her services were no longer required. Galeazzi believes it was the siblings' mother's doing.

Richard went on to marry his adopted first cousin, with whom he had five children.

Drummer Cubby O'Brien joined The Carpenters in 1972 when they were riding high on hits like Close To You. At first, Karen hid her weight loss well, he says. 'It was hard to tell that anything was wrong because Karen wore sweatpants and sweatshirts – layers of clothing. It was only when she was in her evening gowns that you could see she was getting thin.

'She would eat salads, move food around her plate. I think she thought it was the one thing she could control, but we didn't know it was a serious problem. We guys in the group would never say anything. I think Richard knew and was worried for her. Friends like Olivia Newton-John, Petula Clark and Dionne Warwick did talk to her, girlfriend to girlfriend.'

Karen's weight dropped to 5st 7lb, according to the documentary, and audiences gasped when she walked on stage, fearing she might be suffering from cancer.

'I think it was a form of attention-seeking, even when it became negative, and people were saying 'You don't look good.' She was able to control something in her life – Richard controlled a lot, her mum controlled a lot, the record companies controlled a lot, and this was one of the few times in Karen's life when she made a stab for independence. She was a rebel in many ways,' says Schmidt.

Another pressure was the duo's squeaky-clean, all-American image in the rebellious era of Vietnam protests, the Watergate scandal and hard rock.

'Because we were brother and sister, we were ridiculed up, down, backwards and forwards... Nobody would go after the music, but they criticised our clothes, our hair... they criticised our audiences because families came,' Karen says in one of the newly discovered interviews.

Unknown to most people, Richard had his own problems. Suffering insomnia, panic attacks and depression, he became addicted to sleeping pills and once fell asleep at his piano. It was an addiction he finally kicked after going into rehab.

Further unhappiness, for Karen, came from her inability to find lasting love. But, in 1980, it seemed she finally had her happy ending when she married self-described property mogul Tom Burris.

Her former aide Maria Luisa Galeazzi remembers bumping into Karen shortly before the wedding: 'She was so thin that I had to look at her twice, but she seemed happy and invited me to her wedding shower and to the ceremony. She was beautiful on the day, but she didn't look happy.'

It later emerged how, three days before her wedding day, Burris told his future wife, who wanted children, that he'd had a vasectomy and would not get it reversed.

'If I'd have known what a jerk her husband was, I'd have dragged her off the altar,' says Galeazzi.

Unsurprisingly, the marriage didn't last. In 1982, the newly separated and painfully thin Karen flew to New York and put herself in the hands of psychotherapist Steven Levenkron, against the wishes of her family, who felt she could be 'fixed' by a Beverly Hills doctor.

She arrived in New York with 22 suitcases of clothes and shoes and checked in to a hotel with a girlfriend for support,

During treatment, Karen confessed to Levenkron that she could take up to 90 laxatives at once and admitted taking up to ten thyroid medication pills a day. Later, it was learned that she had also been unknowingly poisoning herself with overdoses of a syrup that induced vomiting.

Karen made some progress in understanding the roots of her illness in intensive therapy sessions but still failed to put on enough weight. Levenkron suspected his patient was still secretly taking laxatives and, as a new tactic, asked Karen to change into a bikini and look at herself in a mirror. 'Karen was skeletal, but she didn't see anything wrong. She thought she was gaining weight,' explains Lucy O'Brien. 'That's body dysmorphia – the thinner Karen was, the fatter she felt.'

By the autumn of 1982, Karen weighed 5st 5lb and, when she told the psychotherapist that her heart was 'beating funny', she was admitted to hospital. Over the next seven weeks she gained 20lb from intravenous nutrition and eating small meals. She believed she was cured, filed for divorce and headed back to Los Angeles. She joined a reunion of Grammy award winners in January, but it was her last public appearance. She was found dead at her parents' home on February 4, 1983. She had suffered congestive heart failure due to complications from anorexia.

Tragically, drummer Cubby O'Brien remembers her optimism and determination in her final days: 'I spoke to Karen just a few days before she died and she was in good spirits. She was in the studio and recording. She was feeling good.'

'She didn't want to die,' says documentary maker Randy Schmidt. 'She told people she was 'going to beat this thing' and she was looking forward to the next stage of her life.

'I think if she'd had the opportunities that people with eating disorders have now she would still be here and would have contributed so much more to music,' adds Lucy O'Brien. 'The lesson is that there is help available and some of that is down to Karen.'

• Karen Carpenter: Starving For Perfection is being screened at film festivals and its creators are in talks with streaming companies for wider distribution. Lead Sister, The Story Of Karen Carpenter, by Lucy O'Brien, is published by Nine Eight Books, price £22.