After 54 years together, they decided to die together...

It was, said their family, a very civilised way to die. Sir Edward and Lady Downes, a shared lifetime of personal and professional triumphs behind them, held hands for their final moments together before climbing on to separate beds to drink the clear liquid containing a fatal dose of barbiturates. Within 10 minutes, watched by their weeping family, they were dead.

The decision of one of Britain's greatest conductors and his wife to end their own lives in a manner, time and place of their choosing has reignited the debate over assisted suicide in Britain. Yesterday, as their children were interviewed by Metropolitan Police officers over their role in facilitating their parents' death, the extraordinary finale to the life of Edward, 85, and Joan Downes, 74, began to emerge.

News of their suicide last Friday was released in a statement by their son Caractacus and daughter Boudicca. It said: "After 54 happy years together, they [our parents] decided to end their own lives rather than continue to struggle with serious health problems. They died peacefully, and under circumstances of their own choosing, with the help of the Swiss organisation Dignitas in Zurich."

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The reluctant choice of Britain's finest post-war conductor – who worked his way up from humble beginnings to lead the world's greatest orchestras, a roll-call of achievements which included taking the baton for the first performance at the newly built Sydney Opera House – was made after his wife of 44 years was diagnosed with terminal secondary cancer in the liver and pancreas earlier this year.

In recent times she had become not only his constant companion, accompanying him to dress rehearsals at the beloved Royal Opera – where they met more than half a century ago when she was an aspiring young ballet dancer and he an emerging conductor – but also his full-time carer.

Sir Edward, although not terminally ill, had had to cope with deteriorating eyesight for many years. By the end of his professional life, when he was well in to his eighties, he was forced to conduct only those pieces contained within his prodigious memory. A recent decline in his general condition, following a hip replacement, and for a musician the desperate blow of losing his hearing, had made the prospect of life without Lady Joan untenable. According to her British doctors, the couple had, at best, a few months left together, though Lady Joan might have only lived for a few more pain-racked weeks. Medication worked intermittently and a delay could have meant she would have been too ill to travel, the family said.

Research on the internet led them to contact the controversial Dignitas clinic, where more than 117 Britons have gone to take advantage of Switzerland's liberal laws on assisted suicide. "It was very calm and very civilised," explained Caractacus. "It is a very good way to go and you are under control. When faced with a situation like that, having control over your end is a very important thing, and my parents were very keen on that."

He added: "So while it was shocking when I found out, it seemed a completely reasonable thing to do and we had no trouble supporting what they did. I don't understand why the legal position in this country doesn't allow it."

Son and daughter had persuaded their parents to allow Caractacus to accompany them from their home in Greenwich, south-east London, to Zurich last Tuesday. Boudicca, 39, a UN worker based in Rome, met them there. Only a small group of close friends, one of whom is a solicitor, was privy to the decision. "In the end, they were relieved we were there," said Mr Downes, 41, an IT worker and part-time musician.

Sir Edward had remained active very late in life, still studying for a Russian degree to keep his brain "ticking over", but the ravages of old age were becoming intolerable. "It was frustration upon frustration for him," explained his son.

In order to be allowed to proceed, Sir Edward and Lady Joan had to provide evidence of their conditions and persuade doctors that they were fully aware of and committed to taking their own lives. "They both lived life to the full and considered themselves extremely lucky to have lived such rewarding lives both professionally and personally. Our parents had no religious beliefs and there will be no funeral," the couple's children said.

Those in the world of music in which the Downes – known to all as Ted and Joanie – had been leading figures for some six decades, expressed extreme shock at the sudden announcement of their deaths. Few appeared to know that Lady Joan, who gave up professional dancing to become a choreographer and later a TV producer before spending the final years of her life as Sir Edward's personal assistant, had been ill. Fewer still were aware that Sir Edward was losing his hearing.

His agent Jonathan Groves described their suicides as a "typically brave and courageous decision". He added: "None of us were aware this was going to happen until after they had died. It was very typical of the way [Sir Edward] lived his life. I do not think there is anyone anywhere who has lived his life with more self-determination than Ted did."

Antonio Pappano, music director of the Royal Opera, where Sir Edward spent much of his career first as a horn player, then as a répétiteur and prompter before becoming a conductor, paid tribute to his "overwhelming knowledge". He said: "Ted and Joanie Downes were the loveliest of people and incredibly supportive of my music directorship."

Roger Wright, controller of BBC Radio 3 and director of BBC Proms, said Sir Edward would be remembered for creating "unforgettable" music events over 40 years. "He was a wonderful musician and will be warmly remembered by audiences and musicians alike," he said.

Suicide is no longer a crime in England and Wales, but aiding and abetting a person's self-inflicted death remains a criminal offence, punishable by up to 14 years in prison. Attempts to change the law have proved unsuccessful. Lord Falconer's Lords amendment to the Coroners and Justice Bill fell last Tuesday, the same day that Sir Edward and Lady Joan left for Zurich.

Sarah Wootton, chief executive of Dignity in Dying, which campaigns for a change in the law, said this latest high-profile case showed there was a need for urgent action. "This problem is clearly not going to go away; we are descending down a slippery slope towards unregulated assisted dying abroad, at a rapid pace," she said.

But for the Downes family, the law had nothing to do with it. "It would not make any difference," insisted Mr Downes. "Even if they arrest us and send us to prison, it would make no difference because it was what our parents wanted."

Dignitas: British deaths

* At 23, injured rugby player Daniel James, became the youngest Briton to be assisted in his suicide. He suffered a collapsed spine in a scrum which left him paralysed from the chest down. He travelled to the Dignitas clinic on 12 September 2008 and died in the presence of his parents.

* Former docker Reg Crew, 74, took his life in January 2003 in front of TV cameras. He had suffered from motor neurone disease for over four years. Before drinking water laced with barbiturates, he told broadcasters that he'd "enjoyed a good innings".

* In 2004 Gordon Hurst's family called for the clinic to be closed. Mr Hurst, 76, of Hitchin, Hertfordshire, had Parkinson's disease. Relatives only heard of his suicide plans through letters that reached them after his death. One finished: "Be happy for me because now I am free of all pain."

The maestro: The life and musical times of Sir Edward Downes

17 June, 1924: Born in Birmingham. He took up the piano and violin aged five.

1952: Joins the Royal Opera to become an assistant to the Czech conductor and composer Rafael Kubelik.

1955: Meets his future wife Joan, a dancer.

1970: Becomes Australian Opera's music director, conducting the first performance at the Sydney Opera House, Prokofiev's War and Peace.

1980: Principal conductor of the BBC Philharmonic.

1983: Chief conductor of the Netherlands Radio Orchestra.

1991: Knighted; becomes associate music director at the Royal Opera.

2005: Final appearance at Royal Opera.

2009: Commits suicide at Dignitas clinic.