In a few hours Auschmoneit will have committed assisted suicide, lying on a single bed in a small suburban apartment. Tomorrow his body will be cremated and later his ashes will be scattered on water, as he has instructed.

If you didn’t know the purpose of the elderly German’s visit you would never guess it from his demeanour. Nothing about Ernst-Karl Auschmoneit is melancholic. In fact the man is so engagingly jocular, so strangely carefree, that you have an instinctive urge to whisk him into a corner and beg him to change his mind. He is wearing a suit and tie today, which somehow makes sense, since the whole point of this ritual is that he dies with dignity.

His face breaks into a wide smile as he greets, like an old friend, Ludwig Minelli, who has been waiting in the crowd with an empty wheelchair and who will take him to his final destination. The two men shake hands. There is polite banter between them but no sentimentality and certainly no morbid allusions to why they are here. What will happen next is, in both their opinions, a blessing and a mercy.

Aschmoneit’s grip is strong as he shakes my hand and his resolve even stronger. He decided three months ago to take his own life and he stands by his decision. “I’ve had a very good life but now it is over,” he says, in a matter-of-fact way, as if talking about selling his car. What is most striking about him is that he does not look ill; a little pale, perhaps, but that is to be expected for an 81-year-old who has just stepped off an international flight.

Does he feel depressed? “No, no. I feel like a weight has been lifted off me. I slept for only three hours last night but that was because it is so painful to lie down,” he says. “It is like lying on bone. Sometimes the pain is close to unbearable.”

Aschmoneit, a former science journalist, was told he had Parkinson’s disease four years ago. For the past three years he has lived in an elderly people’s home in Mölln. But he can still walk a little with the aid of a stick and shows none of the uncontrollable shaking normally associated with Parkinson’s. This does not, in honesty, look like a man for whom living is unbearable. The fact that his mind is razor sharp only emphasises the fact that this is going to be a very premature death.

Yet this is his point. He knows that before long the muscles will waste away completely in his legs, buttocks and chest, then he will be bedridden and dependent on a full-time carer to wash and dress him, something he describes as a “vision of horror”. If he waits much longer, travelling abroad and arranging his own death may no longer be an option.

It’s difficult to think of a more excruciating irony. He wants to kill himself while he is still well enough.

Aschmoneit’s wife died from dementia seven years ago. They had no children. He was appalled as he watched her protracted, humiliating demise, punctuated by her repeated pleas that he should help her to end her life. He did not. In Germany it is legal to help a person to commit suicide, but you are then legally obliged to call an ambulance immediately so that the person might be resuscitated. If not you risk being prosecuted for violating “guarantor duty”.

A few months ago when his illness had reached the stage where he could no longer write or read properly — his dearest passions — Aschmoneit joined Dignitas, a non-profitmaking Swiss charity which is unique in offering assisted suicide not only to native citizens but also to foreigners. Assisted suicide has been legal in Switzerland since 1942 provided there is no financial gain or selfish motive.

Dignitas was founded in 1998 by Minelli, a somewhat eccentric-looking lawyer and former journalist, and since then 145 people — mostly German — have died in that fourth-floor apartment, usually by swallowing a lethal dose of pentobarbital-natrium, a barbiturate that renders them unconscious in two to five minutes and dead not long afterwards. Only one British person, a man suffering from throat cancer, has died there so far. The charity now has 1,081 members worldwide. So popular is the “service” that Zurich is rapidly gaining a reputation as the suicide capital of the world, an image that does not sit well with some politicians.

Minelli, 70, who helps to run the charity from his modest home in Forch, Zurich, is highly critical of governments such as Britain’s that make it punishable by imprisonment to help the terminally ill to end their lives painlessly. “It is not hard for me to meet someone who is going to be dead in a few hours because it is what they want,” he says. “We will all die in the end. What I find hard is understanding why governments force their subjects to leave the country to seek help or make them so desperate that they throw themselves under locomotives or off buildings. That is a cruelty. It is Nazism in reverse. The Nazis forced people to die miserable deaths. They force people to live miserable lives.”