Kidman told Urban: 'Its rehab or the marriage is over'

Urban allegedly admitted to Kidman over the phone that he had fallen off the wagon and started drinking again.

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Kidman found out about Urban's problems after flying to Europe to film 'His Dark Materials: The Golden Compass'.

And with his wife away working Keith, 39, started to hit Nashville's bar scene again.

Kidman was tipped off by friends that Keith was out drinking, and called to confront him.

"Nicole called Keith and confronted him," an insider told Life & Style magazine.

"He got defensive and accused her of spying on him."

But Urban eventually admitted his drinking to Kidman, and she flew straight back to the US to be by his side.

"She gave him a choice. He could enter a program or she'd leave him," a friend of Kidman's said.

"He chose the former. Keith can't stand the thought of life without Nicole.

"That's his motivation to kick this thing for good."

And the pal says that Keith's problems have had a massive impact on their marriage.

"She's an emotional wreck," the friend said.

"She would never say it to Keith, but she's telling her family and close friends that her marriage is hanging by the thread.

"She says she wants to help Keith conquer his demons so the two of them can move on with their lives."

Kidman has received support during a difficult time from ex-husband Tom Cruise and his fiance Katie Holmes.

"As soon as Tom heard, he called Nic to offer any help he could give.

"He said to call anytime for anything she needs," a friend of Kidman's told the magazine.

"It meant a lot to Nicole."