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When prince was a frog

It wasn’t always a fairy-tale romance.

When the royals were on the rocks, Prince William showed a more common side — that of a cad who dumped Kate Middleton by cellphone, and then celebrated by getting hammered in a pub, according to an explosive new book.

Until the heir to the British throne popped the question in October by slipping his late mother Princess Diana’s ring on Kate’s finger, William was sometimes more frog than prince.

PHOTOS: PRINCE WILLIAM & KATE MIDDLETON

In the upcoming book, “William and Kate: A Royal Love Story,” a copy of which was obtained by The Post, author Christopher Anderson details their volatile, behind-the-scenes breakup in April 2007.

It began with the rumor of a revived affair with love rival Jecca Craig.

“Do you know who I am!” William shouted in one conversation to Kate, while denying the affair, according to the author. “No one tells me what to do. I do what I want.”

On April 11, William called it off — in a cellphone call. “For the next hour, you heard Wills sputter the reasons he was breaking up with her. ‘I don’t want you suffering the way my mother did,’ ” he reportedly told her, referring to the way his mother Princess Diana died in a car crash while being chased by paparazzi in 1997.

” ‘I can’t,’ he stuttered ‘It just isn’t going to work. It isn’t fair to you.’ ”

But William’s lament quickly turned to liquor.

The prince and his band of bar hopping buddies show up at Mahiki. “As the Rolling Stones classic ‘You Can’t Always Get What You Want’ blasted over the sound system, Wills waved his arms in the air and shouted, ‘I’m freeeee!’ ” according to the book. ” ‘Let’s drink the menu.’ ”

Three months later, the prince asked for a reconciliation.