On Wednesday, 104-year-old scientist David Goodall bid farewell to his home in Australia to fly across the world to end his life.

The lauded ecologist and botanist is not suffering from a serious illness but wishes to bring forward his death. Key to his decision, he says, has been his diminishing independence.

"I greatly regret having reached that age," Dr Goodall said on his birthday last month, in an interview with the Australian Broadcasting Corporation.

"I'm not happy. I want to die. It's not sad particularly. What is sad is if one is prevented."

Assisted dying was legalised by one Australian state last year following a divisive debate, but eligibility requires a person be terminally ill. It is illegal in other states.

Dr Goodall says he will travel to a clinic in Switzerland to voluntarily end his life. However, he says he resents having to leave Australia to do so.

Active life

The London-born academic had lived on his own in a small flat in Perth, Western Australia, until only a few weeks ago.

He stepped back from full-time employment in 1979, but remained heavily involved in his field of work.

Among his achievements in recent years, Dr Goodall edited a 30-volume book series called Ecosystems of the World and was made a Member of the Order of Australia for his scientific work.

In 2016, aged 102, he won a battle to keep working on campus at Perth's Edith Cowan University, where he was an unpaid honorary research associate.

Accompanying Dr Goodall on his journey out of Australia on Wednesday was his friend, Carol O'Neill, a representative from assisted dying advocacy group Exit International.

Mrs O'Neill said the dispute in 2016 over Dr Goodall's working space had affected him greatly. The row began when the university raised concerns about his safety, including his ability to commute.

Although Dr Goodall ultimately prevailed, he was forced to work in a location closer to home. It came at a time when he was also forced to give up driving and performing in theatre, Mrs O'Neill said.

"It was just the beginning of the end," she told the BBC.

"He didn't get to see the same colleagues and friends any more at the old office. He just didn't have the same spirit and he was packing up all his books. It was the beginning of not being happy any more."

Dr Goodall's decision to end his life was hastened by a serious fall in his apartment last month. He was not found for two days. Later, doctors said he needed to engage 24-hour care or be moved into a nursing home.

"He's an independent man. He doesn't want people around him all the time, a stranger acting as a carer. He doesn't want that," Mrs O'Neill said.

"He wants to have intelligent conversation and still be able to do the same things like catching the bus into town."

Divisive debate

Switzerland has allowed assisted suicide since 1942. Other countries and jurisdictions have passed laws allowing people to voluntarily end their life, but many state terminal illness as a condition of eligibility.

The Australian Medical Association (AMA) remains strongly opposed to assisted dying, which it sees as an unethical practice of medicine.

"Doctors are not trained to kill people. It is deep within our ethics, deep within our training that that's not appropriate," president Dr Michael Gannon said during last year's legislative debate in the state of Victoria.

"Now, not every doctor agrees with that," he added. Indeed, a survey of the AMA - Australia's most influential medical association - found four in 10 members supported right-to-die policies.

Mrs O'Neill said Dr Goodall's main desire was to die peacefully and with dignity.

"He's not depressed or miserable, but there's just not that little spark that was there a couple of years ago," she said.

An online petition raised $A20,000 (£11,000; $15,000) for the scientist to fly in business class to Europe. He will visit family in France before heading to Switzerland with his closest relatives.

"They [my family] realise how unsatisfactory my life here is, unsatisfactory in almost every respect," Dr Goodall told the ABC. "The sooner it comes to an end, the better."

Mrs O'Neill said he had spent recent days revising his final letters and holding conversations with his extended family, including his many grandchildren.

Dr Goodall's story has gained attention locally at a time when his home state, Western Australia, considers whether to debate assisted dying legislation.

The state government has publicly expressed sympathy for Dr Goodall, but said any proposed legislation would cover only terminally ill patients.

"My feeling is that an old person like myself should have full citizenship rights including the right of assisted suicide," Dr Goodall said last month.

He told ABC he hoped the public would understand his decision, saying: "If one chooses to kill oneself then that's fair enough. I don't think anyone else should interfere."

Where else is assisted dying allowed?

Assisted suicide describes any act that intentionally helps another person kill themselves, for example by providing them with the means to do so, most commonly by prescribing a lethal medication.

It differs from euthanasia, which is a third-party intervention to end a life to relieve suffering, such as when a doctor administers the lethal dose.

■  In Switzerland, assisted suicide is allowed only if the person assisting acts unselfishly. It is the only country with centres offering assisted suicide to foreign nationals

■  The Netherlands, Belgium and Luxembourg permit euthanasia and assisted suicide. In the Netherlands and Belgium, euthanasia is available to minors in specific instances

■  Colombia allows euthanasia

■  Six US states - Oregon, Washington, Vermont, Montana, California and Colorado - permit assisted dying for terminally ill patients. The US capital Washington DC implemented a similar law for the city's residents in 2017   

■  Canada followed the province of Quebec in permitting euthanasia and assisted suicide in 2016