It was an emotional moment for John Prescott when he found about the "courage" of one of his ancestors
Former Deputy Prime Minister John Prescott has been given evidence his three times great-grandfather fathered children with his own daughter.
Mr Prescott, who was born in Prestatyn, Denbighshire in 1938, makes the discovery in a BBC Wales programme.
Genealogists traced his Welsh ancestry back 300 years.
He is moved to tears when he learns his great-great grandmother, Athaliah Parrish, bore four children while living with her widowed father, Thomas.
Mr Prescott's family moved from Wales to Yorkshire when he was aged five.
In the series, Coming Home, he returns to Chirk, in the county of Wrexham, to research his Welsh roots further.
His Welsh mother, Phyllis, was from the border town, where she and her 11 siblings lived in a miner's cottage.
John Prescott visited Chirk and Prestatyn to find out about his ancestors
With the help of genealogist Gillian Smith he learns of their working "below stairs" for the aristocracy.
But as the roots of his family tree are probed further, the secret about his maternal great, great grandmother, Athaliah Parrish, is revealed.
Ms Smith tells him: "From what I've been told, the daughters took over the roles of the mother when they died, and they took over all the roles of the mother. It was quite common as well."
Athaliah later married neighbour William Jones. Thomas Parrish was not invited to the wedding.
She had consumption and died aged 42 in 1868. She left 10 children.
Mr Prescott said: "The evidence it clear, though we can't prove it 100%, that my great, great, great grandfather, Thomas, fathered the children with my great, great grandmother."
He said he admired Athaliah's "courage".
He said: "It's something that people might say you don't want to talk about, but I didn't think I would get emotional about this.
"I just think, she was a wonderful woman."
In 2007, Mr Prescott announced his intention to stand down as Hull East MP at the next election.
Coming Home will be broadcast on BBC One Wales (Monday 30 November, 1930 GMT).