An aristocrat has won his legal battle to allow his son born by surrogacy to inherit the £157million family fortune.

Ceawlin Thynn, the 8th Marquess of Bath and owner of the Longleat estate, had his second son with wife Emma, Marchioness of Bath, in 2016 via a surrogate after she suffered from a rare inflammatory condition during her first pregnancy.

Henry Thynn was born to a surrogate mother in America, raising concern about his eligibility to benefit from three family trusts because of the historical definition of a legitimate child.

The nine-year-old is genetically the son of the 51-year-old Marquess - also known as Lord Bath - and his wife, Lady Bath.

But the family trusts use a pre-1970 common law definition of 'children' that predates modern fertility treatments.

At the High Court Lord Bath had asked for a 'blessing' to allow Henry to potentially inherit a share of the fortune. 

On Thursday Mr Justice Matthews said in a ruling that it would be 'unfair on Lord Bath and on Henry' to treat the boy as though he were not his father's son.

'Henry is the son of Lord and Lady Bath. Not only is he treated by them and will be treated by the world in general as a child of Lord and Lady Bath's marriage but he is also their genetic child,' he added.

Though his older brother John, 11, would inherit before him because of primogeniture - which provides for a firstborn legitimate child to inherit their parents' estate - it was argued Henry could still stand to benefit.

The judge said at this stage the trustees only wanted the power to add Henry as a beneficiary but not to exercise it yet.

Mr Justice Matthews said this delay was to 'avoid any problems with US tax' because Henry was born there to an American surrogate mother'.

'A decision can then be taken at a later stage, in the light of appropriate advice, whether to exercise the power to add him,' the judge added.

Henry is believed to be the first member of the British aristocracy to have been born by surrogacy, after his mother suffered hypophysitis during her first pregnancy in 2014.

That pregnancy came a year after she married her husband, who inherited his title seven years later following the death of his father from Covid aged 87.

Lord Bath's parents had snubbed the couple's 2013 wedding, held at Longleat, with his father expressing great displeasure over his son's modernisation of the estate, which included removing a number of his father's prized erotic paintings from the main house.

The colourful late Marquess once claimed to have about 70 girlfriends he referred to as 'wifelets'.

Until the pandemic the current Marquess was known as Viscount Weymouth, as he had been since taking control of the Longleat estate in 2013.

His wife, then styled Viscountess Weymouth and now Lady Bath, became the country's first black British marchioness when she gained the title in 2020, a year after she appeared on Strictly Come Dancing, reaching the seventh week with her partner Aljaz Skorjanec.

Born in the capital, she studied History of Art at University College London before taking a course at the London Academy of Music and Dramatic Art (Lamda).

After a stint in Los Angeles she returned to the UK to become a presenter and chef.

Her husband runs the 900-acre Longleat estate on the Wiltshire-Somerset border and told the Daily Mail after Henry's birth: ''Never did I imagine that in West Hollywood I'd become father to John's little miracle baby brother.

'It's a wonder of modern science that the Longleat Bath family has been completed - for now at least - by Emma and I having a much-loved son, helped so crucially by a tremendous surrogate in California, to extend our family.'

His wife, also a former fashion model, said: 'We are simply ecstatic. His arrival has completed our little family and brought us so much happiness.

'We have certainly been worried about how people will react to the news.

'I just want them to know this is not about my vanity or that I was too lazy. I'm not the kind of person who would have done this for anything less than a very important reason.

'I didn't care about my weight gain or that breast feeding would ruin my body. I just want to live to see my children grow up. I did not want to take the risk of something tragic happening. I really enjoyed being pregnant up until the point when the terrifying pains began.'

Doctors warned having a second child could kill Lady Bath after the brain illness she suffered while pregnant with John in 2014.

She was initially diagnosed with a disorder of the pituitary gland, which left her with pain she described as 'like a knife stabbing at my brain'.

'I had never known such pain,' she told The Mail. 'It was beyond anything I could imagine.'

She added: 'I tried everything to alleviate it, but often ended up lying in a dark room trying not to move. Even breathing too hard was agony.'

The headaches – which started in the summer of 2014, during her third trimester – were manageable with painkillers until the family was on holiday in the South of France. There, her symptoms escalated.

'I was more worried about the baby than myself,' she continued. 'It was so upsetting to be that ill. It hurt so much that I threw up and Ceawlin called an ambulance.'

An MRI scan revealed a bleed on her brain.

Although doctors initially suspected a non-cancerous tumour, she was diagnosed with a rare but serious condition called hypophysitis, involving swelling and bleeding, which can cause a stroke during a 'traditional' birth.

Lord Bath added: 'The doctors didn't understand hypophysitis very well at all but they knew it was potentially fatal.

'They had to inject Emma with steroids to help develop the baby's lungs, as they prepared for an emergency C-section three weeks before her due date, because her life was at risk from a natural birth.

'It was a very worrying time, especially as no one could really tell us much about the condition.'

She underwent months of tests even after John was born safely.