A mother has admitted giving her terminally ill seven-year-old son a large dose of morphine to stop his suffering and “quietly end his life”.

Antonya Cooper, from Abingdon, Oxfordshire, said her son Hamish had stage 4 cancer and was in “a lot of pain” before his death in 1981.

Now facing a terminal diagnosis herself, she made the admission to BBC Radio Oxford as part of an effort to change the law on assisted dying.

Police said they were investigating her case. Assisted suicide – intentionally helping another person to end their life – and euthanasia – deliberately ending a person’s life – are illegal in England.

Hamish had neuroblastoma, a rare cancer that mostly affects children.

He was five when diagnosed and was initially given a prognosis of three months.

Following 16 months of “beastly” cancer treatment at Great Ormond Street Hospital, his life was extended but he was left in great pain, according to his mother.

She said: “On Hamish’s last night, when he said he was in a lot of pain, I said: ‘Would you like me to remove the pain?’ and he said: ‘Yes please, mama.’

“And through his Hickman Catheter, I gave him a large dose of morphine that did quietly end his life.”

BBC Radio Oxford asked the 77-year-old if she believed her son knew she was intending to end his life.

She replied: “I feel very strongly that at the point of Hamish telling me he was in pain, and asking me if I could remove his pain, he knew, he knew somewhere what was going to happen.

“But I cannot obviously tell you why or how, but I was his mother, he loved his mother, and I totally loved him, and I was not going to let him suffer, and I feel he really knew where he was going.”

She continued: “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.”

Asked if she understood she was potentially admitting to manslaughter or murder, she replied: “Yes.”

“If they come 43 years after I have allowed Hamish to die peacefully, then I would have to face the consequences. But they would have to be quick, because I’m dying too,” she added.

Four decades after Hamish’s death, his mother is coming to terms with her own incurable cancer.

She said his suffering and her own ill health had cemented her feelings on assisted deaths.

“We don’t do it to our pets. Why should we do it to humans?” she said.

Campaigners for a so-called “right to die” have argued that people should be able to choose when and how to die in order to avoid suffering.

Critics have said changing the law would “place pressure on vulnerable people to end their lives” for fear of being a financial or emotional burden.

MPs recently discussed the issue at a parliamentary debate, at which the government said it was a matter of conscience for individual parliamentarians rather than one for government policy.

In a statement, Thames Valley Police said it was “aware of reports relating to an apparent case of assisted dying of a seven-year-old boy in 1981”.

It added: “At this early stage, the force is making inquiries into these reports and is not in a position to comment further while these investigations continue.”

This is a hugely complex and highly controversial subject, and yet it is one that is gaining momentum.

Assisted dying is the phrase used to describe a situation where someone who is terminally ill seeks medical help to obtain lethal drugs which they administer themselves. Assisted suicide, is helping another person end their life.

Both are illegal in the UK but recently, Scotland, Jersey and the Isle of Man all announced they are considering changing the law to let terminally ill people end their lives.

One hundred and ninety cases have been referred to the Crown Prosecution Service over a 15 year period. Most were not taken forward, there have been four successful prosecutions.

If you have been affected by any of the issues in this story, the BBC Action Line has links to organisations which can offer support and advice