Nicole Kidman is getting even with her ex-hubby Tom Cruise for flaunting his love affair with Penelope Cruz — by cavorting with Brit pop bad-boy Robbie Williams!

Nicole and Robbie, 27 — who shot to stardom in the mid-1990s as part of the band Take That — first set tongues wagging this past September when they teamed up on a duet of ‘Somethin’ Stupid,’ a remake of the 1967 hit by Frank Sinatra and his daughter Nancy.

And the pair “really turned up the heat on romance rumors in mid-October when Robbie sneaked into the Dorchester Hotel in London to pay a late-night visit to the red-haired siren after she did a talk-show taping,” an insider revealed.

“Robbie didn’t emerge from the hotel until 3 a.m., looking ruffled and smiling.

“Then the twosome reportedly spent the night together following his Royal Albert Hall concert, secretly meeting at a casino before continuing their tryst elsewhere.

“After that, the couple spent seven hours locked in a London studio filming a sexy video to promote the ‘Somethin’ Stupid’ single, in which they trade steamy kisses.”

Later in the torrid video — which has created a big stir in the British press — Nicole brandishes a whip and ends up wearing little more than a diamond necklace and a strategically placed purse.

“Most recently, the pair spent two hours alone at Sydney’s posh Establishment Hotel following his concert there,” said the insider.

“Then he followed Nicole back to her Darling Point home. And the following day, Robbie was a guest on Nicole’s yacht anchored in Sydney Harbor.

“Nicole and Robbie are really smitten with each other.”

When asked what it was like to make beautiful music with the swaggering singer, Nicole, 34, responded coyly to a reporter: “Call it chemistry, if you like.”

For his part, Robbie took a swipe at Nicole’s diminutive ex when he told the reporter: “I was amazed at how well our voices worked. I think the fact that we’re the same height really helped, too!”

Said the insider:

“By her outrageous behavior with Robbie, Nicole is sending a clear message to Cruise — two can play at this game.”