Winehouse is nominated for her number one album Back To Black

The R&B star has been overtaken by relative unknown Bat For Lashes as the punters' favourite for the award.

"Amy's had a difficult time of it lately, and that's coincided with backers deserting her," said Ladbrokes spokesman Nick Weinberg.

Winehouse, 23, has spent several spells in rehab over recent weeks amidst reports of drug use.

She has also cancelled a string of concerts, festival appearances and award show performances.

Her soul-tinged Back To Black album was previously joint favourite to take the Mercury prize alongside Bat For Lashes' Fur And Gold.

Bat For Lashes is the stage name of singer Natasha Khan

But Winehouse has slipped over the past week. Her odds now stand at 11/4, compared to 9/4 for Bat For Lashes.

The eventual winner, who receives �10,000, will be chosen by a panel of experts at a ceremony on 4 September.

Organisers say they still expect Winehouse to attend the show.

Video no-show

Meanwhile, the singer's latest video has had to be filmed in her absence.

She sings lead vocals on New York hip-hop producer Mark Ronson's single, Valerie.

But producers had to get a cast of extras to take her place at the video shoot, which took place in London this week.

Although tabloid reports claimed Ronson was furious at her absence, film-makers told the BBC they had known all along that Winehouse would not be able to appear.

Ronson produced half of the tracks on Winehouse's album.