A loving mum “may have hastened” the death of her terminally ill son when she gave him a morphine overdose, an inquest heard.

Antonya Cooper’s son Hamish had stage 4 cancer when she administered the large dose of morphine sulphate in 1981. Police launched an investigation more than 40 years later when she admitted helping him to “quietly end his life”.

But assisted dying campaigner Antonya, 77, died before any charging decision was made. The inquest into Hamish’s death was told there was no firm evidence to say the morphine overdose killed him.

He had endured 16 months of “horrendous treatment you wouldn’t put a dog through” after being diagnosed with neuroblastoma in 1979. Thames Valley Police launched a probe after Antonya spoke of helping her son to die in a BBC radio interview last May.

She died two months later after battling terminal breast, liver and pancreatic cancer. Speaking about her beloved son’s final moments at home in December 1981, Antonya said he was in a lot of pain.

“We had watched him brave through all that beastly treatment,” said the former chair of Neuroblastoma UK. “We had had him for longer than the original prognosis, so the time was right. It was in the middle of the night, we were by his bedside.

“I had been entrusted to the possession of that morphine sulphate. On Hamish’s last night, I said, ‘Would you like me to remove the pain?’ He said, ‘Yes please, mama’. And through his Hickman Catheter I gave him a large dose of morphine that did quietly end his life.”

Antonya, from Abingdon, Oxfordshire, said that she believed her son knew what was going on as she ended his life. “I was his mother, he loved his mother and I totally loved him,” she added.

“I was not going to let him suffer, and I feel he really knew that he was going. It was the right thing to do. My son was facing the most horrendous suffering and intense pain. I was not going to allow him to go through that.”

Coroner Darren Salter said neither a post-mortem examination nor a toxicology report were available regarding the cause of Hamish’s death. In a statement, retired doctor Patricia Pinches – who certified Hamish’s death – said Antonya claimed Hamish had “died after going to sleep”.

Dr Pinches said: “She said she was relieved and had I been in her position I would have been too.” The family’s GP, Dr Peter Tate, said he believed “no line was actually crossed in relation to the treatment of Hamish”.

He added that there was “never any feeling of any wrongdoing” and said he never spoke to Antonya about an overdose. The coroner returned a narrative conclusion at Oxford coroner’s court, saying: “Hamish’s death may have been hastened by morphine administered to him.”

Euthanasia is illegal in England and could be prosecuted as murder or manslaughter. A proposed law to legalise assisted dying succeeded in an initial vote in Parliament in November. It could see terminally ill adults in England and Wales with under six months to live legally allowed to end their lives.

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