Brad Pitt's (search) overwhelming desire to have children sunk his marriage to Jennifer Aniston (search), after the "Friends" star decided to put motherhood on hold to pursue her acting career, a pal of the power couple said.

Pitt will now move out of the couple's $13.5 million Beverly Hills mansion, as Aniston keeps the home as the base of operations for her campaign to continue with a full schedule of big-screen rolls.

A pal said Aniston doesn't want to take the time off to have a kid — and she doesn't want to endure the physical effects that giving birth will have on her sexy body.

"It's about the children," the pal said. "She just doesn't want kids right now — and he wants kids."

The mega-couple still love each other despite the split, a source said.

They came to the decision that their nearly five-year marriage was over during a tense huddle in their hotel on the Caribbean island paradise of Anguilla (search), where they were vacationing with friends David Arquette and Courteney Cox Arquette (search).

Aniston's publicists at the PMK agency planned to break the news to the world yesterday, when they'd be out of the office and could conveniently dodge reporters' calls for two more days.

But Pitt's flacks at Baker Winokur Ryder, another L.A. agency, didn't play along.

When reporters called Pitt's spokeswoman, Cindy Guagenti, her typically straightforward statements suddenly turned vague.

When The Post told Guagenti that it intended to go ahead and report that a divorce was imminent, the couple issued its stunning statement to People magazine.

"We would like to announce that after seven years together, we have decided to formally separate," it began.

Pitt and Aniston returned to the Van Nuys, Calif., airport from their final trip together at 5:19 p.m. Saturday.

What they found was a country aflame with news of their blockbuster breakup.

The ill-fated couple had spent most of 2004 busy plying their acting craft in far-flung locations, sometimes going whole months without seeing each other.

The strain on the marriage could be seen right on Aniston's finger, which was bereft of her wedding ring for much of the past year.

The issue has always been Pitt's desire to be a father and Aniston's desire to remain childless.

Initially, she said she wanted to wait until her popular sitcom, "Friends," had finished its 10-year run.

But when that happened early last year, she started filling her schedule with new projects, not ob/gyn appointments.

On the set of the forthcoming "Mr. and Mrs. Smith," Pitt took an immediate shine to sexy co-star Angelina Jolie.

The former "Sexiest Man Alive" was not craving Jolie's pillowy lips and "Tomb Raider" physique — but time with her 3-year-old son, Maddox.

Pitt spent hours in Jolie's trailer playing with the tot, sources said.

New confidante Jolie advised Pitt to stay with Aniston.

"A friend said that he [told Jolie] that he wanted to leave Jen, and Angelina said 'Don't do it,' " said an industry insider.

A report in the News of The World newspaper in Great Britain yesterday said the last straw in the couple's troubled relationship came when Aniston overheard a call on the couple's house phone in which Pitt and Jolie talked "as lovers do . . . in hushed tones."

But a source told The Post Jolie was not a factor in the breakup, which the source said was solely over the couple's differing views on parenthood.

Pitt has been surprisingly public about his frustration with Aniston's career-first approach to family planning.

"We've been in rehearsals long enough," he told CNN recently.

"It's time, it's time," he told People.

But the problems between the two A-list stars, married since 2000 and reportedly having trouble for almost that long, went deeper than mere fertility. In fact, both Pitt and Aniston have basically admitted they simply are not soul mates.

Last year, he told Vanity Fair, "Jen and I made a pact, that we'll see where this [marriage] is going. I'm not sure it really is in our nature to be with someone for the rest of our lives, just because you made this pact."

If that sounded lukewarm, Aniston told W magazine a year earlier: "Is he the love of my life? I think you're always sort of wondering, 'Are you the love of my life?' He's certainly a big love in my life."

Pitt, 41, and Aniston, 35, have been together since being set up on a blind date in 1998.

They were married two years later at a blockbuster, $1 million Malibu wedding.

But Pitt never lost his taste for excitement.

"He's a smart-ass guy. He loved hanging out with the guys from 'Oceans Twelve' and doing the whole bachelor thing with George Clooney," said one friend.

"But Jennifer is a stay-at-home girl, a nice, boring person," the friend added.

Other sources said Aniston was fed up with Pitt's tinkering.

The couple bought a Beverly Hills home several years ago, but couldn't move into it for two years because Pitt, who fancies himself an architect, was personally overseeing the extensive renovations.

And yet that project might have actually kept them together temporarily.

"Finishing that house was the worst thing that could have happened," said one friend. "They needed another project."

The breakup appeared to be cemented during the couple's Caribbean holiday, which many believe was hurriedly set up by the couple's publicists as something of an extended photo-op for a happy marriage.

It worked for a few days. Most of the celebrity magazines sported large photos of Pitt and Aniston embracing and joking around.

"Brad and Jen Back On!" reported Star magazine.

But their $100,000-a- week Anguilla love nest turned into a little bungalow in a town called Splitsville.

Additional reporting by Gersh Kuntzman