Tai Altman knew for years that, if the time came, his parents planned to kill themselves together. Joanna Moorhead hears the story of a devoted couple who believed in the right to die

For more than two decades, Tai Altman knew exactly how his parents' lives would end. When the moment came to say goodbye, knowing they were about to kill themselves, he felt surprisingly calm. They had prepared him in every possible way, he says: he knew only that he respected their decision and didn't want their last memory of him to be tears streaming down his face. "They were in their bedroom," he says. "Dad was in his chair, Mum was sitting on the end of the bed. I said: 'I love you. Goodbye.' Then I went out of the room, down the stairs and left."

Tai left his parents' home in Oxford that day and spent the night at a hotel in Luton: next morning, he flew home to Inverness. The postman had delivered a letter from his parents. "You know what we've been planning and the time has come," he read. "Call our GP and tell her to let herself into the house; the side door is open."

For Raphael and Tamar Altman, sending that letter was the penultimate act of their lives. The final one was to swallow a lethal draught they had bought on the internet. When their doctor, alerted by Tai, entered their bedroom, she found Raphael, 69, dead in his chair and Tamar, 72, beside him on the bed with her head on his shoulder and her arm across his chest. In their diary for that day they had written one word: "Depart".

Last week, four months on, the inquest into the Altmans' deaths concluded with a verdict of suicide. Today, Tai, 43, is remembering his parents and telling their story, and his own. He doesn't want them to be remembered solely for the manner in which they died and yet it's clear that their end was rooted deep in their lives. For many years, much of their thought and focus was on the way it would occur. From all Tai says, it's clear his parents had to work very hard – especially at the end – to ensure that not only would they die as they chose, but that he and his sister Tanyah would not be implicated in their deaths.

Tamar and Raphael before illness struck.

Raphael and Tamar's marriage was, says Tai, a union of opposites: he was quietly solid, an instinctive intellectual, she was chatty, arty and passionate. Both were raised in Jewish families, Tamar in New York City and Raphael in Cape Town. Neither was a believer, but Judaism was culturally important to them and they met in Israel after both gravitated there in their teens.

It was the 1960s and the couple seemed to have been made for hippydom: in Tai's photograph albums they smile out from under long, flower-strewn locks and gaze at one another as they strum guitars in their flared trousers. For a while they lived in a commune in Formentera, then Tamar got pregnant and they moved to London so she could give birth before heading back to the beach. In naming their son, they consulted the I-Ching, the book of changes, an ancient Chinese classic text, for inspiration: Tai means "peace approaching".

Later, they returned to Britain where, now a family of four, they moved into a caravan in the grounds of the country-house headquarters of a philosophical commune, the School of Economic Science. "They were living the Sixties dream," says Tai.

Tai Altman in his father Raphael's arms in the early 60s.

When he was five and his sister Tanyah was three, they moved to Great Milton in Oxfordshire, to the house in which, almost 40 years later, they would die. Raphael enrolled at Oxford University as a mature student of theology, working as a cleaner to make ends meet. Tamar worked as an occupational therapist before retraining as a counsellor. At some point in the midst of their busy, bookish, music-filled lives, they became aware of the campaign for assisted suicide and decided it was a cause they very much believed in. They joined several right-to-die organisations including Exit and Dignity in Dying, and Tai, by then in his 20s, realised what was in their minds. "They didn't ever sit me down and spell it out, but there were plenty of comments here and there about the right to choose to die, and Mum made it very clear that if Dad decided to go, she was going too. She said life without him would be a life not worth living."

At the time, though, it seemed unlikely that Raphael would die first. His mum, says Tai, was often ill: she had neuralgia, joint problems, arthritis and eventually diabetes. Her physical problems led to psychological issues such as anxiety; through it all, Raphael was her rock. It was obvious within the family that Tamar couldn't live without Raphael, but no one imagined she would ever have to.

Then, in summer 2006, Raphael was diagnosed with a form of lymphoma called Waldenstrom's Macroglobulinemia. Ever the researcher, he threw himself into finding out everything he could about this rare condition, even as the doctors hooked him up to chemotherapy drips. And as quickly as it had descended, the threat of illness seemed to lift: he rallied, and enjoyed three years of reasonably good health in which he and Tamar cruised on the Norwegian fjords and holidayed in the Lake District. "They knew they didn't have forever and made every moment count," says Tai.

Raphael's cancer returned in 2009, and the following year, buoyed by more chemotherapy, he submitted evidence to the Commission on Assisted Dying. To Tai, now married to Sarah and the father of two little boys, Noah and Leo, the path ahead was obvious. "By this stage he was taking a huge number of drugs – there were drugs to combat the cancer and drugs to combat the side-effects of the other drugs," says Tai. "He had anaemia, he was being given blood transfusions, and his world was diminishing."

So, too, was Tamar's, though more as a consequence of Raphael's ill-health than her own medical condition. In 2012, Sarah gave birth to a surprise third child – Hannah Rose, a first granddaughter for Tamar and Raphael. Tamar doted on her. Tai, despite knowing how committed his mother was to dying with Raphael, suggested to his mother that she could choose to live on alone after his father's death.

"I said that she could come to live with us in Scotland or if she wanted to stay in Oxford she could get a new house that would be easier to run, or we'd help her to stay on in the house in Great Milton," he says. "I said, you don't have to do this – you could see your grandchildren get older.

"But she wouldn't entertain it. She said, 'You've always known what I intend to do. Living without Dad would be abject misery and who would want to live in abject misery?'

"She said the two things she would be saddest to leave were her garden and Hannah Rose, but she was definitely going. Much as I thought she didn't have to go, I didn't think I should beg her to stay either. The final decision was hers."

Tai Altman, his wife Sarah and their children on the last family visit to his parents, Raphael and Tamar, at their Oxford home, summer 2013.

Everyone in the Altman family knew last summer would probably be Raphael and Tamar's last; Tai and Sarah took the children to Oxford to see their grandparents and they have photographs of the couple sitting in their garden smiling at Hannah Rose and her brothers playing at their feet. Raphael hadn't given up hope of a new drug or a treatment trial that could help to keep him alive, but he was now heavily involved in planning the end: Tai would receive email after email telling him which estate agent they should use to sell the house, where to find bank information, and which magazine subscriptions would need to be cancelled.

One morning in October, Raphael woke up with a tingling sensation in his arm. He was sent to hospital for tests and admitted. Tai flew down to visit and was shocked by how quickly his father was going downhill. With each hour that passed he seemed able to do less: he was losing control of the left side of his body. The doctors suggested more tests, but the family said they wanted to take him home.

Next day, in Great Milton, Raphael saw a stream of friends, spoke to people on the phone, dictated emails. "It was time to say goodbye and he didn't want to leave anyone out," says Tai.

Amid the tears, there was room for humour. "Some of his believer friends sent cards about what was waiting on the other side and we had a few chuckles about that," he says.

Tamar, meanwhile, had retreated into herself. "She was in a strange mood. She saw a few girlfriends and she was with my dad a lot. She spent a bit of time eating the sorts of food she couldn't usually eat because of the diabetes – ice cream, cereal – because now it didn't matter any more."

Raphael's condition was deteriorating rapidly and it was clear that if he was going to swallow the poison himself, which he had to do that so no one else could be held legally responsible for his death, it would have to be soon. That night, they ordered in an Indian meal ("Mum didn't eat it, but the rest of us did"); and next morning Tai and Tanyah said farewell.

At this point, says Tai, the hardest point of his journey began. The reason he has chosen to share his story in such detail is this: like his parents, he believes it should be legal to assist people who are determined to end their own lives. Because it is not, he and Tanyah had to make sure they were well clear of their parents' home before they died. "It was a huge worry, leaving them alone. They had bought the drugs on the internet; what if they weren't what they purported to be after all? What if the dose worked for one and not the other? What if my father spilled the potion as he was trying to drink it?"

Were the law different, he says, he would not have left them alone: although he wouldn't have stayed as they died. "It was a moment for them to be together, just the two of them," he says. "They were devoted to one another, they were two halves of a whole. It was the end of their love story."

For Tai the next 24 hours, until the police confirmed that his parents were dead, were filled with anxiety. Once he heard they had fulfilled their plan he was hugely relieved and he realised, he says, that because they had prepared him so well for what was to come, he had already done his grieving. Due to his father's meticulous planning, he and Tanyah could organise the funeral knowing exactly what they wanted (wicker coffins and interment in a natural burial ground) and the house was cleared out and sold within weeks. "They worked hard to make it as easy as possible," he says. "It was the last big thing they cared about, that things would be OK for us."

He used to talk to his parents on the phone every day, now he misses them every day. "They were my sort of people. Mum loved watching winter sports and when I see something about the Olympics I think, I want to talk to her about that. Or Hannah Rose says or does something and I think, Mum would have loved hearing about that."

The truth is, he says, that time had run out for Raphael. For Tamar it was different. "He didn't really have a choice because his life was ebbing away – but she did." And now, at those times she would have enjoyed, Tai allows himself a moment of sorrow that she's not around. Then he remembers that, for all the joy Hannah Rose and the other grandchildren would have brought her, Tamar would have had many hours of loneliness without Raphael. "She made her choice," he says. "She was quite certain about what she wanted and I think she was incredibly brave to do it."