Inside 5 Hertford Street, an exclusive London members' club so secretive it won't even reveal the cost of a membership

Earlier this year Business Insider compiled a ranking of the capital's most exclusive members' clubs, and found that one was particularly intriguing.

5 Hertford Street is so exclusive that it declined to give even a ballpark indication on the cost of membership.

We were instead sent one photo of the bar at its downstairs nightclub called Loulou's, which only left us wanting to see more of what actually goes on down there.

Its Instagram account is also private. But after some digging, we've managed to put together a rare glimpse behind the scenes at the only members' club for those in the know, including Lupita Nyong'o, George and Amal Clooney, Mick Jagger and Leonardo DiCaprio when they're in London, as well as young royals Prince William and Princess Eugenie.

So if you're curious as to where the young, rich, beautiful and connected head to after hours, keep scrollin'.

Perched on the corner of Shepherds market, in London's Mayfair, behind an unmarked maroon door sits this secretive and exclusive private members' club. Royals, billionaires, and A-listers are all known to frequent the club — when they can get a reservation.

It's owned by Robin Birley

Step inside and you will find quintessentially English clashing patterns and prints on the carpets and walls — the club's trademark style — reminiscent of an eccentric upper-class country home. Fashion designer Rifat Ozbek is behind its boho-chic interior.

Eclectic art lines the walls of the various living and sitting rooms which are lit by cabaret-style table lamps. Expect to see members wandering around in smoking jackets, velvet slippers and signet rings.

There are plenty of chandeliers, some more unusual than others.

Almost every room is equipped with a roaring fireplace, offering the perfect spot for a cosy nightcap, or TV dinner.

Source: Vogue.

There are various private dining rooms for long lunches, which dogs are apparently welcome to join.

...Or lavish celebratory dinners by candlelight.

The club has its own cigar shop, naturally. And it appears that guests are welcome to chomp on one while sampling the extensive cocktail list. The gin fizz apparently comes highly recommended.

Source: Vogue.

But the real fun starts down a steep staircase to 5 Hertford Street's famed Loulou's nightclub, one of the best kept secrets of the capital's rich and famous.

Loulou's is renowned for its dimly-lit Parisian Bohemian decor and its giraffe mascot.

Source: Evening Standard

Harry Styles is apparently a fan of the club — when he manages to charm his way through the door, which is apparently not all the time. Here he is posing with Loulou's giraffe.

As are Hollywood A-listers, including Lupita Nyong'o, George and Amal Clooney, Mick Jagger, Leonardo DiCaprio, and Margot Robbie, who are all known to drop in for dinner, drinks or dancing when they're in town.

Source: Evening Standard

Fahionistas favour the venue for launch parties and events. Below, supermodel Bella Hadid attended a Marc Jacobs event at Loulou's.

The young royals including Prince William, Kate and Princess Eugenie, who was snapped on her way in below, are all big fans of Loulou's.

Source: Evening Standard.

It's really not hard to see the attraction of the club's dimly-lit, moulin rouge vibe, where it's easy to imagine all sorts of debauchery goes down after hours.

Even the ladies' loos are a rather dramatic affair, taking boho-chic theme to another level.

Loulou's pièce de résistance is the stunning cocktail bar.

A gin & tonic there will apparently set you back about £20. 5 Hertford Street declined to provide a ball-park figure for the price of membership. Instead, it said that membership is gained through application only and a price given upon request.

Source: Evening Standard