The sharp-tongued X Factor judge loves rubbing shoulders with the rich and famous and boasts of having pop legend Elton John and Duchess of York Sarah Ferguson among her closest friends.

But brother David reveals how, behind their backs, she gossips outrageously and breaks their most intimate confidences. And in the case of Elton, Sharon's secret treachery took a horrible twist.

David told us: "Elton is Sharon's oldest friend. But to this day, he still doesn't know about one truly disgusting act of spiteful revenge she took against him in the early 70s, long before she met Ozzy.

"Sharon and Elton both fancied the same young man in Los Angeles. And when Elton was the one who finally got him into bed Sharon was furious.

"Accompanied by a friend of ours, who told me everything, she sneaked up to Elton's Beverly Hills mansion, defecated on his driveway then used it for her own dirty protest all over his Rolls-Royce.

"It was absolutely appalling. But that's just the way Sharon ALWAYS retaliates.

"Many years later, after she had Ozzy arrested for trying to strangle her, I sent a note telling him to keep his chin up. A few days later, it was hand-delivered back to me in a Tiffany box. I opened the box to discover Sharon had sickeningly used my note as toilet paper."

She pulled a similar stunt after finding a cuddly toy monkey given to her dad by his mistress. "Dad said after soiling the monkey's head, Sharon moved it outside the front door so it would be sitting there to welcome him home," added David "She's got a real problem." Sharon must know that—in her autobiography she even admits once peeing in singer Lynsey de Paul's suitcase!

David, 56, described how his mouthy sister thinks nothing of passing on pals' secrets just to raise a cheap laugh. "Sharon's a gossipmonger through and through," he said. "She opens her mouth to ANYBODY who happens to be there.

"Not even Elton is spared, even though he's her closest mate. He'd be horrified to know she tells everyone about his financial affairs. She's always talking about how he spends too much so always has to work just to pay the bills.

"Worse, she revels in revealing what Elton's told her in confidence about the most intimate details of his relationship with David Furnish. Sharon dines out on those stories—just as she does with those Elton tells about what he got up to with former manager John Reid years ago. But while Sharon adores Elton she just about tolerates his boyfriend David. Elton's a generous friend to her and she knows what side her bread is buttered. She loves holidaying with them at their home in the south of France. Her other great pal is Sarah Ferguson. And Sharon relishes passing on all her juicy secrets."

David, who worked closely with his sister as a producer on her American TV chat show, recalled: "Fergie came on the programme and their friendship blossomed from there. They're now extremely close.

"Fergie's always telling Sharon things in confidence, not realising she loves to spread it about. One of Sharon's favourite stories is the reason Fergie split from her ex, Prince Andrew. According to Sharon Fergie paid him a surprise visit on board ship and caught him in bed with a celebrity lover."

Another of Sharon's best buddies is Little Britain star David Walliams. And her brother reveals she even took his side over her own daughter Aimee. "Walliams had taken Aimee out on a few dates and she'd liked him a lot," said brother David. "But then Walliams stopped returning her calls and Aimee got the needle. Sharon stuck the knife in by inviting Walliams over to their Berkshire home for dinner with her and Fergie. Aimee was so upset she refused to come downstairs—and spent the whole evening sending Walliams nasty texts.

In the end Sharon posed for photos of herself and Fergie having fun with Walliams and he texted them back to Aimee, which only upset and wound her up even more."

Singer Robbie Williams is a favourite with the Osbournes. David said: "Ozzy in particular likes him very much after they met at an Alcoholics Anonymous meeting in LA. Ozzy doesn't go in for lengthy descriptions of people. And he always refers to Robbie as the ‘bum bandit'. He doesn't mean anything by it—it's just a jokey wind-up."

But one celebrity Sharon loathes is Madonna. "Sharon can't stand her since they had an ‘encounter' at an LA restaurant," said David. "Sharon was lunching with a woman who worked with Madonna and the star came over to say hello.

"But when their mutual friend tried to introduce Sharon, Madonna didn't even turn to look. She merely extended her hand for Sharon to shake. Well, Sharon has a mouth like a fishwife and calls Madonna a ‘dirty f***ing c***'. And she hasn't stopped attacking her ever since.

"As Sharon puts it, she thought it was ‘disgusting' when Madonna ‘tried to buy a baby'." Rocker Sting got similar treatment. David said: "Sharon first met Sting and his wife Trudi Styler at a party in 2002. Elton introduced them but Sharon took offence at the ‘Hello' Sting gave her.

"She was convinced the couple were disrespectful and snooty and parried with, ‘Aren't you really a Geordie? Didn't you come from Newcastle?' Of course, he does—although you'd never know it these days.

"The feud continued the next year when Ozzy announced on their TV show The Osbournes, ‘I'm the Prince Of F***ing Darkness. I'm not proud of being a drug addict and alcoholic. I'm not proud of biting the head off a bat or the head off a dove. But it could be worse...I could be Sting.'

"We later heard from Elton how Sting went hysterical about that. But what made matters worse was when Ozzy's quotes were used on a huge hoarding on the road into London from Heathrow. Sting went insane."

The Beckhams aren't Sharon's favourite couple either. "People assume they must be friends because of how she stuck up for Victoria when Becks' mistress Rebecca Loos was on Celebrity X Factor," said David.

"But the truth is Sharon isn't overly keen on either of them. When she went to their pre-World Cup party she and Ozzy left early. Sharon said, ‘It was full of f***ing footballers and their cheap little wives.'"

Courtney Love is high on Sharon's hate list, too. "Sharon can't stand her and calls her a ‘horrible piece of work'," said David. "Jack's always been friendly with Courtney's daughter, Frances Bean, who'd often hide out with Jack instead of going home.

"A distraught Courtney would ring demanding to know where her daughter was but Sharon always refused to take her calls.

"She hates Courtney so much she makes her make-up artist Jude Alcala relay the story of the day he went to Courtney's home on a job.

"According to Jude, Courtney was standing there stark naked when she cocked her leg on the side of the bath and carried on her personal hygiene regime right in front of him.

"Sharon loves demeaning Courtney with that."