LONDON - Scottish singer Leon Jackson is on course to secure the Christmas number one spot with his debut single after winning "The X Factor" at the weekend, retailers said on Thursday.

Mid-week sales figures show that the 18-year-old's cover of the Mariah Carey and Whitney Houston hit "When You Believe" is well ahead of the opposition.

Record chain HMV said Jackson is set to sell up to 300,000 copies of the single in less than a week, making it the year's fastest-selling track.

"Even at this early stage it looks like being Leon all the way," said HMV spokesman Gennaro Castaldo, who collated sales figures from HMV and other shops.

In second place is the current number one, Katie Melua and the late Eva Cassidy's version of the classic Louis Armstrong song "What A Wonderful World".

Last year's X Factor winner Leona Lewis is in third place with "Bleeding Love", while "Fairytale of New York" by the Pogues and Kirsty MacColl has received a boost from the BBC's aborted attempt to censor it.

Bookmaker William Hill has slashed the odds on Jackson from 1-6 to 1-20.

"We need a Christmas miracle to save ourselves a monster payout," said Hill's spokesman Rupert Adams.

Jackson won the fourth series of the talent search show on Saturday. He was given a million pounds recording contract after beating Welsh singer Rhydian Roberts in the final.