A couple grieving over the death of their 20-year-old son are coming to terms with the fact that two children they considered to be their nephew and niece are their biological grandchildren. Their son donated sperm to his aunt and her same sex partner when he was just 15.

Charlie Lowden, died following a routine hernia operation aged 20 last year.

After his death, it emerged that he donated sperm – unofficially- to his aunt Sarah Ashman, then aged 35 and her partner Claire Ashman, ten years her junior.

Ms Ashman subsequently gave birth to a son, Charlton, now aged five. Three years later, the couple had another child with Charlie, a girl named Sarah.

The father’s mother, had assumed for years that the children were her nephew and neice, but Charlie’s death prompted Mrs Lowden’s sister to confess that they were really her grandchildren. Mrs Lowden told the Daily Telegraph: “When our Charles died we were broken hearted. We thought we had nothing left of him. But there is.

“I just wish that we had known about all of this before he died so that Charlie could know we had accepted it.”

She continued: “When Charlie died, Sarah said I had no idea how special he had been to her – but now I do.”

Mrs Lowden said that her son used to hold Carlton up to a mirror and joke “Who’s the daddy? I’m the daddy.” She did not realise that her son was actually telling the truth.

Ms Ashman said: “Charles was very special to me. He was great. When he died, it was our secret. But I had to tell Lynn. I couldn’t go through life not telling her because they are her grandchildren.”

Mrs Lowden told the paper that learning the truth was “just like having our Charlie back”

“We’ve got the next best thing to him and it’s not a secret any-more. I’m absolutely delighted. He did it for a reason and he has left a legacy.”

Although unofficial sperm donation is not illegal, official sperm donors must be aged between 18 and 45.

Charlie died after being discharged from Hexham General Hospital in Northumberland folowing an operation on his hernia. His parents are bringing legal proceedings against the the hospital claiming that he was not given adequate protection from blood clots. The hospital say they have investigated the case and shared with the family the areas of care they will improve following his death. An inquest will be held next month.