A man who took a DNA unwittingly uncovered a huge family secret that his mum had kept hidden for decades.

The 27-year-old said he had always wanted to do an ancestry test because his dad had said they were related to Sir Arthur Conan Doyle, the author behind Sherlock Holmes.

His parents had split up when he was just one-year-old, and when he told them he was doing a DNA test his dad "marvelled at how far along science had come" but his mum quickly urged him to cancel the order and ask for a refund.

At first, he assumed she thought he was wasting his money on something that wouldn't give accurate results, but he soon became suspicious of her behaviour and arranged to take her out for dinner.

A man talking to his father
The man decided to break the news to his dad (stock image)

He questioned her about it and says she "shook me to my core" by admitting that she had never been sure if his legal father is in fact his biological father.

When the results came back, the virtual family tree on his mum's side were 'all expected results' but on his dad's side he saw names he recognised, 'just not my family as I had known it'.

He decided to break the news to the man he had always thought to be his dad, who took it in his stride and 'immediately averted his worst fear' by saying it would do nothing to change their relationship.

But the man says the 'real twist' is who his biological father actually is, saying his mum swears it could only be a man he used to know as his dad's best friend.

He wrote on Reddit: "I grew up with this other guy’s daughters (my apparent half-sisters) and wife as close family friends. Shoot, I even went to their family gatherings sometimes.

"I’m now faced with a quandary related to whether I reach out to my apparent biological father or let sleeping dogs lie.

"My biological dad has been married for 26 years and has all of his stuff together so now I have to decide if I am going to rock his work as hard as mine has been."

And he added: "Fun family anecdote has always been that he rubbed off on me because we share a cleft chin. Makes sense now."

People were left torn over whether they thought he should tell the man he suspected to be his biological dad, but one argued: "Mate, he probably already has a good idea. He slept with your mum at the right time, after all."