X Factor winner Leona Lewis is already the Christmas No1 - hours after her debut single went on sale in the shops.

Copies of the 21-year-old Londoner's cover of the Kelly Clarkson hit A Moment Like This began flying off the shelves at record stores as they opened at 9am today.

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She had already sold 150,000 downloads - meaning that she is now effectively in the top spot.

Lewis, mentored by X Factor judge Simon Cowell, needed to notch up around 200,000 single sales to achieve No1 this Christmas.

Record industry insiders predict she could achieve as many as one million sales by Sunday -far exceeding last year's X Factor winner Shayne Ward's total of 740,000 in his first week.

HMV spokesman Gennaro Castaldo said: "Leona's single is already the fastest selling download in history and is now almost guaranteed to be the fastest selling single of the year.

"There has been a huge response to her as a contestant and when you consider eight million votes were cast then you have to assume a good deal of those people will be buying the song."

He said her sales will be driven this week mostly by mothers and young girls.

"They are not generally in the download market so it will be mums buying copies as stocking fillers for their daughters, perhaps even multiple copies as gifts, that will drive hard single sales this week," he added.

Take That are likely to be in second place for Christmas while McFly - who have a double-A side out - are tipped to be third.

Girls Aloud's cover of Tiffany hit I Think We're Alone Now could edge up to fourth or fifth place, he said.

Take That are poised to be the No1 Christmas album.