One of Britain's most respected conductors, Sir Edward Downes, and his wife, Joan, a choreographer and TV producer, have died at an assisted suicide clinic in Switzerland, their family said today.

Downes, 85, was almost blind when he and his 74-year-old wife, who had become his full-time carer, travelled to Switzerland to end their lives, a family statement released to the BBC said.

Born in Birmingham, Downes had a long and distinguished career, including conducting the first performance at the Sydney Opera House. He worked with the BBC Philharmonic and the Royal Opera House in London.

The statement from the couple's son and daughter, Caractacus and Boudicca, said they "died peacefully, and under circumstances of their own choosing".

The statement continued: "After 54 happy years together, they decided to end their own lives rather than continue to struggle with serious health problems."

The couple died at a clinic run by Dignitas, the Swiss organisation that operates a specialist euthanasia service.

The Downes family said: "Our father, who was 85 years old, almost blind and increasingly deaf, had a long, vigorous and distinguished career as a conductor.

"Our mother, who was 74, started her career as a ballet dancer and subsequently worked as a choreographer and TV producer before dedicating the last years of her life to working as our father's personal assistant.

"They both lived life to the full and considered themselves to be extremely lucky to have lived such rewarding lives, both professionally and personally."

Downes was knighted in 1991.A Metropolitan police spokesman said Greenwich CID had launched an investigation.

"We continue to investigate the circumstances of their deaths. [There are] no further details at this stage," he said.

In the past, police have investigated cases in which British people have travelled to the Dignitas clinic. Anyone assisting a person to end their life could face up to 14 years in prison.

Prosecutors have not pushed forward cases against families and friends of the growing numbers of Britons who have travelled to Dignitas to die, however, and there is fierce debate about whether the law should be changed to protect people from prosecution.

Last December, the Crown Prosecution Service announced it would take no action against the family of 23-year-old Daniel James, who travelled to Switzerland to die after being paralysed from the chest down in a rugby accident.

The police did not investigate the deaths earlier this year of Peter and Penelope Duff, who became the first terminally ill British couple to be helped to die together in Switzerland.

Last week, the House of Lords voted against an attempt by the former lord chancellor Lord Falconer to relax the law on assisted suicide. His amendment to the coroners and justice bill would have allowed people to help someone with a terminal illness travel to a country where assisted suicide is legal.

Debbie Purdy, who has multiple sclerosis, is seeking to clarify the law in the House of Lords. She wants a ruling that her husband will not be prosecuted if he helps her travel abroad to die.

Some people fear that relaxing the law on assisted suicide would lead to an increase in cases, and put people at risk of being pushed into taking their own lives. Gordon Brown is against a change in the law.