PARTY-lover Hugh Grosvenor is the new Duke of Westminster — inheriting £9.9 billion ($16.71 billion) at 25.

He becomes Britain’s third-richest man following the death of landowner dad Gerald, 64, The Sun reports.

Single Hugh now owns 300 acres of London and a string of Downton-style country estates.

A pal said: “He’s really down-to-earth.”

Hugh is happy mixing with royals but never acts the toff with his other friends, they insisted tonight.

Baby-faced Hugh is Prince George’s godfather and once threw a £5 million ($8.44 million) 21st birthday bash attended by Prince Harry and other A-listers.

However his super-rich parents were determined he enjoyed a normal childhood, and sent him to a state primary school.

He went on to study at Newcastle University and has recently been working as an account manager at London eco-coffee firm Bio-bean.

But after the death this week of his father Gerald, 64, he is now Britain’s third-richest man — and the 68th wealthiest in the world.

Despite the family’s vast wealth, pals told how the new Duke of Westminster enjoyed a grounded upbringing thanks to Gerald and his duchess wife Natalia.

They wanted a “normal” childhood for Hugh — whose title is “Earl Grosvenor” — and his three sisters Lady Tamara, 36, Lady Edwina, 34 and Lady Viola, 23.

The Duke hated his time at posh boarding school Harrow and decided to send his son to a primary school near the family’s Eaton Hall home in Cheshire.

He went on to attend private day school Ellesmere College in Shropshire, before getting a 2.1 in Agriculture at Newcastle University.

Previously unseen pictures see Hugh boozing with a string of girls while dressed in a pink onesie at Newcastle’s Club Trop in 2011.

Two years beforehand he can be seen goofing around with pal Ben Lamont and a blow-up doll on a skiing holiday in the Swiss resort of Zermatt.

The photos are in stark contrast to his royal life — where he is best mate to Harry and William.

A friend said: “You’d never have known that his dad was a billionaire.

“He never flashed the cash although he would often put his hand in his pocket to buy a round of drinks.

“He’s a really down-to-earth bloke who never acted like a toff and is just a really good guy.

“His dad had been ill for the past couple of years and had lost an awful lot of weight — he was painfully thin and seemed to be wasting away.

“I know Hugh, and the rest of his family, will be devastated by his death.”

The Grosvenor family have long-lasting links with the Royal Family and the Queen wrote to them privately after Gerald died at the Royal Preston Hospital on Tuesday.

He is believed to have suffered a heart attack at his grouse shooting estate, Abbeystead House in Lancs.

He was the Queen’s 14th cousin once removed and she asked him to mentor Prince William, whose godmother is Natalia.

The Prince of Wales and Duchess of Cornwall were also said to be “deeply shocked and greatly saddened” by the unexpected death.

A spokesman for William and Kate said: “The Duke and Duchess of Cambridge were very sad to learn of The Duke of Westminster’s passing.

“Their thoughts are very much with his family.”

Hugh’s sister Lady Edwina is married to TV historian Dan Snow, while Lady Tamara is wed to Prince William’s pal Edward van Cutsem.

It is the Duke’s only son who now inherits 300 acres of central London, including parts of Belgravia and Mayfair, a string of country estates and tracts of land in Scotland, Oxford and Cheshire.

It is unclear if Hugh will give up his current job as an account manager at Bio-bean, which recycles coffee grounds.

Pals have always said he is “immensely private”.

However, he did hit the headlines in January 2012, when he held a lavish 21st birthday party which reportedly cost £5 million ($8.44 million) and featured an array of A-listers.

The celebration, which was held in a marquee in the gardens of his parents’ estate, was attended by 800 guests including Prince Harry.

Comedian Michael McIntyre and hip-hop stars Rizzle Kicks also performed.

The dress code was “black tie and neon”, a “light show” went off at midnight and there was free-flowing champagne.

During the event, guests were reportedly treated to a firework display and a dinner featuring a “tasting plate” of half a Scotch egg, smoked salmon and Morecambe Bay potted shrimps for starters.

The main course was steak and chips and dessert was served near the bar area.

At the time, Hugh said: “The party was simply amazing — a birthday and a party I will never forget.

“It is the beginning of a new era in my life and I look forward to the challenges that lie ahead.”

He now has a few more in front of him.

WHY HUGH GETS TITLE

The ancient law of primogeniture means the late Duke of Westminster’s eldest kids cannot inherit his title as they are female.

So it leapfrogs Lady Tamara and Lady Edwina.

The late Duke also leaves his widow, Dowager Duchess Natalia, 57, and youngest daughter Lady Viola.

LANDED GENTRY

The Seventh Duke of Westminster is only 25 but he is worth £9.9 billion and owns more of Britain than the Queen.

Hugh Grosvenor has inherited 133,100 acres including 300 acres in exclusive parts of London such as Belgravia and Mayfair, and become the country’s third richest person.

His portfolio includes the family seat, the 11,500-acre Eaton Estate just outside Chester.

He also owns the 23,500-acre Abbeystead shooting estate in Lancashire, where his dad Gerald collapsed on Tuesday.

Hugh has 96,000 acres of Reay Forest in northwest Sutherland, Scotland, home to three peaks and up to 3500 wild deer.

The family’s 1800-acre Halkyn Estate in North Wales is rich in limestone, church stone and lead.

The quarrying of these commodities is what created the family’s original wealth.

La Garganta in southern Spain also goes to Hugh. Princes William and Harry have been known to hunt on the agricultural land.

The legacy continues with development company Wheatsheaf, property company Grosvenor and the Westminster Foundation — the Grosvenor family’s charity giving arm.

And Hugh takes over the Grosvenor art collection, which owns works by painters including Rembrandt, Van Dyck and Sir Winston Churchill.

They are sometimes displayed at the Grosvenor Museum, Chester.

This article originally appeared on The Sun and was reproduced with permission.