JANET JACKSON is hiding a shock­ing secret from her fans – a $20 million wedding to her billionaire fiance, WISSAM al MANA, The ENQUIRER has learned exclusively.

And in an even more stunning development, a renowned Turkish author claims that the 46-year-old beauty has secretly converted to Islam!

In a 2010 interview with a Turkish television net­work, Adnan Oktar (aka Harun Yahya) – who wrote the controversial anti-evolution tome “Atlas of Creation” – in­sisted that Janet is a practicing Muslim, as was her late brother Michael.

“I sent that nice Janet Jack­son a copy of my book,” he said. “I also sent her late brother a copy. Her brother was a Muslim. Janet is also, in fact, a Mus­lim. Like her brother, she keeps it hidden.

“She is a devout believ­er…like her brother.”

Although Janet was raised a Jehovah’s Wit­ness, she revealed in a 2001 interview that she had stopped practicing organized religion. These days, the performer is highly secretive about her religious beliefs, say those close to her.

“I think Janet knows that the American public as a whole doesn’t think too favorably of the Is­lamic religion,” a source told The ENQUIRER. “It’s highly unlikely she will ever publicly address her religion.”

But fans would be hard-pressed to miss the extreme lifestyle changes Janet has undergone over the past few years.

The woman who made headlines after her breast was exposed during a 2004 Super Bowl half­time performance with Justin Timberlake has been dressing much more conservatively since her relationship with 37-year-old Wissam became serious, and she no lon­ger exudes the same level of sexuality she did in her earlier years, notes the source.

In fact, last December, she was spotted at the Los Angeles International Air­port wearing traditional Muslim women’s clothing – a black jilbab (a long, loose-fitting garment) and a black hijab (head scarf ). And in another show of modesty, Ja­net remained completely covered under a beige, full length cover-up while at the beach during a vacation in Sardinia, Italy, in July – while Wissam was shirtless.

According to Muslim tradition, women are ex­pected to hide their beauty from strange men by cov­ering themselves from head-to-toe with long robe-like garments.

And Janet might be surprised to learn that she may have to share her husband! Some of the more radical followers of Islam believe men are allowed to have up to four wives.

Additionally, to conform to Muslim cultural norms, Janet may have to give up some of her western grooming habits. She won’t be permitted to pluck her eyebrows or wear false eyelashes, wigs, weaves or hair extensions – all of which are forbidden in Islamic culture.

But those things ap­parently won’t stop her from her dream wedding, and wealthy Wissam is sparing no ex­pense. Expected to take place in Doha in the tiny Arab country of Qatar – where he was born – it will be “one of the biggest, most memorable wedding extravaganzas in recent history,” said the source.

Janet and Wissam, whose family empire has business interests that range from real estate to media, met after the “Nasty” singer performed at a private concert in the Middle East in December 2009. They reportedly got engaged early last year when the handsome ty­coon presented Janet with a stunning 15-carat dia­mond ring. Although the couple had originally con­sidered tying the knot in late 2011, sources say they decided to wait so they’d have more time to plan the festivities.

“They have tenta­tively set a late 2013 wedding date,” confirmed the source. “But they are still working out the details and exact day.”

Right now, the luxu­rious St. Regis hotel in Doha is at the top of the cou­ple’s list of locations for the nuptials, says the source – and Wissam is expected to spend a whopping $3 million to fly in the approximately 500 guests from all over the world on pri­vate jets.

The guests’ accommoda­tions – a week’s worth of food, lodging and activities – will set him back more than $2 million. The reception will likely cost another $5 million, and entertainment – which will include the biggest stars of both the Arab and west­ern worlds – will run around $3 million.

Wissam also plans to set up a mini-boutique at the hotel, where guests can shop for high-end fashions from top designers like Versace, Gucci, Dolce & Gabbana and Hermes. That will cost another $2 mil­lion.

And to thank guests for at­tending the wedding, each will receive a $10,000 Rolex watch, bringing the tab for party fa­vors to a whopping $5 million!

“Wissam is going to splash big when he marries Janet,” added the source. “It’s going to be the wedding of the cen­tury.”

But Janet has already made some sacrifices as Wissam’s wife.

“The lifestyle that Janet has with Wissam is a far cry from the celebrity culture that she’s accustomed to, where she was idolized and was seen as a powerful woman,” noted the source.

“Right now, Janet is living a double life. While in the United States and Europe, she still plucks her eyebrows and wears hair extensions. But when she’s visiting Wissam’s family in the Middle East, she doesn’t put on makeup or hair extensions, and she wears tra­ditional Muslim clothing.

“It seems as if Janet has lost all sense of her old self. She is madly in love with Wissam, and willing to do almost any­thing to make him happy.”