Spears has sparked fears for her mental health

The singer, 25, left a treatment centre in Malibu, California, on Wednesday, less than 24 hours after her manager told reporters she had checked in.

Spears made headlines at the weekend by shaving her head in front of onlookers shortly after a separate stay in rehab.

Her estranged husband, Kevin Federline, has asked for an emergency hearing in the couple's ongoing custody battle.

It is not known what issue the 28-year-old aspiring rapper plans to raise, but the hearing is due to take place in Los Angeles family court on Thursday.

"Our best information is he will be here; she won't," said court spokesman Allan Parachini, "but anything is possible".

Spears had her first number one hit when she was 17

The couple currently share custody of 5-month-old Jayden James and 17-month-old Sean Preston, following a temporary custody agreement made in January.

Since splitting with Federline last year, Spears has been a constant feature of gossip columns with her partying lifestyle.

On Tuesday, celebrity website People quoted a statement by the star's management which said: "Britney Spears has voluntarily checked herself into an undisclosed rehab facility today."

Manager Larry Rudolph asked for privacy while she received treatment, but the singer was later photographed on the balcony of a clinic in Malibu, whose previous clients include Robert Downey Jr and Matthew Perry.

A spokesman for the public relations agency that represents the treatment centre said he had no comment and would not confirm that Spears had been there or that she had left.

Hair auction

Last week, she reportedly sought treatment at a separate centre in the Caribbean but also checked out after a day.

The next day, she arrived at a San Fernando Valley hair salon, grabbed an electric razor and shaved her head bald.

Afterward, she visited a tattoo parlour, where she had a pair of red-and-pink lips inked onto her wrist.

Potential bidders can view Spears' hair on the salon's website

Then she went out clubbing with friends before apparently checking into rehab for a second time.

The salon where Spears had her drastic haircut has set up a website to auction the hair for more than $1m (�512,500).

A spokeswoman for the website said they had already received a number of bids.

"We have received many offers and we are currently in the process of separating the real ones from the bogus ones," she told the BBC News website.

Spears has sold more than 50m albums in her 8-year career, but has not released any new material since 2004.

Editor of Vibe magazine, Jon Caramanica, said it was hard to say whether the star's recording career had been damaged by her recent troubles.

"I think the best thing she could do is make an album that engages with this subject matter in some way," he said.

"Not only to have a good album but to have an album that's a declaration of emotional health."