She may not have royal blood — but she’s worth a king’s ransom!

Prince William is romancing a beautiful billionaire heiress — the daughter of Virgin Atlantic Airways and Virgin Music business baron Sir Richard Branson!

Royal insiders say it’s William’s first serious romance after a series of flings. And he’s so keen on keeping his relationship with Holly Branson under wraps that he’s throwing up a smokescreen by popping up in public with other young beauties.

But a royal insider disclosed: “Holly is Wills’ No. 1 — no question.

“He loves the way Holly isn’t a bit impressed by who he is — she just likes him for himself. After all, she’s had a pretty privileged upbringing herself.”

Holly’s billionaire dad Richard is the flamboyant founder of the Virgin Group companies, which have holdings in everything from fashion, hotels and air transport to books, films and music — even Virgin Cola. He was knighted in 1999.

Despite that, pals of the 20-year-old Prince say Holly is unspoiled, the sort of brainy beauty — she’s studying to be a doctor — that William falls for.

“They’ve known each other for four years, and saw each other on and off, but the romance has really blossomed recently out of that friendship,” said a Buckingham Palace source.

“William found himself spending more and more time with Holly because they had so much in common. She’s interested in sports and so is William. They like the same kind of music and movies, they both have an artistic side — and most important, they both have a great sense of humor.”

The couple has spent time together both at Prince Charles’ estate at Highgrove and in London, with Charles’ blessing, insiders say.

And recently, William was Holly’s most important guest at her 21st birthday party at her father’s country home near Oxford.

Sources say William learned a painful lesson when his mother Princess Diana, after her split with Charles, saw her serious romance with surgeon Hasnat Khan end because he couldn’t deal with the media spotlight.

“William is afraid his romance with Holly will also wither and die in the glare of media attention,” said the insider.

“So he makes a point of being seen with other eligible girls because he knows it will throw the press off the track.

“Recently he was out shooting with pals at the Queen’s country estate of Sandringham, and the press was speculating on who the pretty girls were and which one was William’s.

“And earlier this year, he was seen in the company of Arabella Musgrave, the 20-year-old daughter of the manager of a polo club not far from Highgrove.”

Holly raised eyebrows four years ago when, at age 16, she modeled her father’s fashion line in a women’s magazine and posed facedown on a bed topless.

And she told the magazine about her ideal man: “He’s tall, about 6 feet 2 inches, with blue eyes and a chunky build, and definitely someone sporty.”

William to a T!