The parents of a 16-year old boy have discovered that he secretly donated sperm to his aunt's lesbian partner so they could have children.
Charlie Lowden knew his aunt Sarah Ashman and her partner Claire wanted to have children, and unofficially offered to donate his sperm. They agreed to keep his identity secret.
Claire, now 30, from Choppington in Northumberland, subsequently gave birth to a boy, Carlton, now five.
Three years later the couple asked him to donate again, which he did, resulting in Sarah, who is now two.
Charlie's parents Charles and Lynn are now coming to terms with the fact that the two children they considered to be their nephew and niece are actually their biological grandchildren.
They only discovered the family secret last December after Charlie died tragically young, following a routine hernia operation at the age of 20.
Speaking in the Daily Telegraph, Mrs Lowden, 52, said: '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.
'When Charlie died, Sarah said I had no idea how special he had been to her – but now I do.'
Despite Carlton being 'the spitting double' of his father, Mrs Lowden said she remained 'quite oblivious' to the truth.
Even though Charlie used to grab the boy, hold him up to the mirror and laugh, 'Who's the daddy? I'm the daddy', Mrs Lowden still did not realise what he was saying.
Sarah, who entered a civil partnership with Claire last year after 13 years together, 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 described knowing Carlton was her grandson as 'just like having our Charlie back'.
'I'm absolutely delighted. He did it for a reason and he has left a legacy.'
Charlie, a scaffolder, died last December after being discharged from Hexham General Hospital in Northumberland following a hernia operation. His parents claim he was not given protection against blood clots and have launched legal proceedings.
Jim Mackey, chief executive of Northumbria Healthcare NHS Foundation Trust, said: 'Mr Lowden died of a rare complication of surgery which we have fully investigated.
'Our report, which we have shared with Mr Lowden's family, showed areas of care where we felt we could improve and we have now implemented those improvements.'
A pre-inquest hearing is due to take place on January 20.
"Unofficial" sperm donation between individuals is not illegal, a spokesman for the Human Fertilisation and Embryology Authority said.
But not using a licensed clinic means the donor remains the legal father, he warned.
When carried out officially, sperm donors should be between 18 and 45.