David Lloyd George's 30-year mistress

David Lloyd George and his mistress Frances Stevenson were 'together' for 30 years

To some he was the greatest figure in Welsh history. A man who inspired Britain in World War One and was regarded as the architect of the welfare state.

But for Ruth Longford, David Lloyd George is considered a grandfather - a legacy of one of the most astonishing tales of extra-marital affairs involving a British prime minster.

Her mother is said to be the illegitimate child of 'The Welsh Wizard', still the only Welshman ever to become the prime minister.

As Ms Longford's grandmother Frances Stevenson was mistress of 30 years and private secretary to the politician.

It is the sort of scandal that today would end a political career. But a century ago, it remained on of the House of Commons' best-kept secrets.

And even when the philandering politician had a love child when leader of the Liberal party in 1929, the cover-up to seemingly protect Lloyd George's reputation was as elaborate as it was incomprehensible.

Frances Stevenson and her 15-year-old daughter Jennifer stand in the doorway of Ty Newydd, the house where Lloyd George died when Jennifer was 16

Only Ms Stevenson's name was on baby Jennifer's birth certificate and the new mother put her new-born up for adoption - before promptly adopting her back.

All seemingly to keep the name of the MP for the Caernarfon Boroughs away from any formal paperwork.

"My mother wasn't completely sure who her father was," Ms Longford admitted to BBC Radio Wales' Lloyd George and his Women programme.

Not only because of Lloyd George's absence on the official documents. There was another twist.

Ms Stevenson was, herself, having an affair. Army officer Colonel Thomas Tweed, who also worked in Lloyd George's office, could have been the father.

But Ms Longford insisted: "My mum was brought up by Frances and Lloyd George.

"She loved him to pieces. She'd prefer him to be the father than the possible alternative."

In his programme tracing the life of his great-great-grandfather, historian Dan Snow acknowledged Lloyd George "was the kind of man that fell in love three times a day."

The young lawyer had married Margaret Owen, the daughter in a well-off farming family, aged 25 in 1888 - just two years before he started his 55-year career as an MP.

He had numerous affairs - even with wives of fellow Liberal MPs - before settling down with Ms Stevenson in 1914.

She had been a personal tutor to Lloyd George's daughter Megan and was a governess at Wimbledon Boarding School before being invited to become his private secretary and most trusted political confidante.

"She was 22 and he was 49 when they met," said Ms Longford.

"He said if you work for me, I won't be able to avoid making you my mistress. So she knew the terms and at that time it was hard to get very interesting jobs.

"She had been totally bored being a teacher in a small girls' school and suddenly she was going to be private secretary to the Chancellor of the Exchequer - where all of the action was. She was very thrilled by that.

"So it was a double attraction - he was attractive and the life she was suddenly imagining before her was going to be really interesting. I could see how that would be amazingly attractive.

"She did have a huge crush on him. She might have been able to resist if he hadn't told her he needed her. But she came running."

Within two years of the affair beginning, Lloyd George was leading Britain's war effort as Secretary of State for War and was about to become PM.

But there was another twist. His lover was pregnant.

"My grandmother was very discreet about it," said Ms Longford.

"But she once bitterly said to my mother, precautions don't always work. There's a point in her diary where she is talking about the excitement of having his son and then she's saying we've agreed it cannot be.

"She made it stop happening. She never openly said 'yeah, I had an abortion' but we think there were a couple of instances where she definitely had to stop something."

While the private secretary and the politician continued their affair in hullaballoo of London, Lloyd George's wife continued bringing up the family in the Gwynedd village of Llanystumdwy on the Llyn Peninsula.

Ms Stevenson was at Lloyd George's side when the British premier negotiated peace at the Treaty of Versailles following the allied victory in World War One.

Lloyd George resigned from Number 10 in 1922 as the coalition Government crumbled.

The next challenge was the prospect of a love child - and this one Ms Stevenson was desperate to keep.

"Categorically she definitely seemed to have planned it," said Ms Longford.

"Frances was in her 40s and Lloyd George was not in power and things were not going particularly well.

"She must have known she didn't have much more time for a child and she wrote to him 'if what we hope happened, has happened I'll let you know with no exchanges' which sounds like she's telling him she must be pregnant.

Ms Stevenson became Mrs Lloyd George following his first wife's death during the Second World War. But within 15 months, the man she lovingly called 'LG' was dead.

And with it went one of Whitehall's most unbelievable episodes.

Lloyd George and his Women is on BBC Radio Wales on Sunday, 11 December at 12:30 GMT