We always joked dad looked nothing like his parents - then we found out why

During the pandemic, Matthew started looking for answers to niggling questions about his family history. He sent off a saliva sample in the post to be analysed.

The genealogy company entered his record into its vast online database, allowing him to view other users whose DNA closely matched his own.

"Half of the names I'd just never heard of," he says. "I thought, 'That's weird', and called my wife to tell her the old family joke might be true after all."

Matthew then asked his dad to submit his own DNA sample, which confirmed he was even more closely related to the same group of mysterious family members.

Matthew started exchanging messages with two women who the site suggested were his father's cousins. All were confused about how they could possibly be related.

Working together, they eventually tracked down birth records from 1946, months after the end of World War Two.

The documents showed that one day after his father was apparently born, another baby boy had been registered at the same hospital in east London.

That boy had the same relatively unusual surname that appeared on the mystery branch of the family tree, a link later confirmed by birth certificates obtained by Matthew.

It was a lightbulb moment.

"I realised straight away what must have happened," he says. "The only explanation that made sense was that both babies got muddled up in hospital."

Matthew and the two women managed to construct a brand new family tree based on all of his DNA matches.

"I love a puzzle and I love understanding the past," he says. "I'm quite obsessive anyway, so I got into trying to reverse engineer what had happened."

Stories of babies being accidentally switched in hospital were very rare at the time, though more are now coming to light thanks to the boom in genetic testing and ancestry websites.

The day after Jan Daly was born at a hospital in north London in 1951, her mother immediately complained that the baby she had been given was not hers.

"She was really stressed and crying, but the nurses assured her she was wrong and the doctor was called in to try to calm her," Jan says.

The staff only backed down when her mum told them she'd had a fast, unassisted delivery, and pointed out the clear forceps marks on the baby's head

"I feel for the other mother who had been happily feeding me for two days and then had to give up one baby for another," she says.

"There was never any apology, it was just 'one of those silly errors', but the trauma affected my mother for a long time."

Matthew's father, an insurance agent from the Home Counties, was a keen amateur cyclist who spent his life following the local racing scene.

He lived alone in retirement and over the last decade his health had been deteriorating.

Matthew thought long and hard about telling him the truth about his family history but, in the end, decided against it.

"I just felt my dad doesn't need this," he says. "He had lived 78 years in a type of ignorance, so it didn't feel right to share it with him."

Matthew's father died last year without ever knowing he'd been celebrating his birthday a day early for the past eight decades.

Since then, Matthew has driven to the West Country to meet his dad's genetic first cousin and her daughter for coffee.

They all got on well, he says, sharing old photos and "filling in missing bits of family history".

But Matthew has decided not to contact the man his father must have been swapped with as a baby, or his children – in part because they have not taken DNA tests themselves.

"If you do a test by sending your saliva off, then there's an implicit understanding that you might find something that's a bit of a surprise," Matthew says.

"Whereas with people who haven't, I'm still not sure if it's the right thing to reach out to them - I just don't think it's right to drop that bombshell."