Television's biggest night of the year ended with the soul singer Alexandra Burke winning The X Factor three years after being dropped from the competition.

The 20-year-old from north London beat the boyband JLS in the TV talent competition to land a £1m record deal.

She had appeared on the show three years go only to be dropped by Louis Walsh, one of the judges, before singing in the final rounds.

On hearing last night’s result, Burke, who was mentored by the Girls Aloud singer Cheryl Cole, said: “Thank you so much for making my dream come true. I’m the happiest girl alive.”

Earlier in the night, Burke had been joined in a duet by the American R&B star Beyoncé Knowles.

Knowles’s performance of her hit song Listen reduced Burke to tears and proved a coup for the show’s bosses.

At least 12m viewers were expected to watch the season finale of the ITV show while 11m were tuning in to see the semifinals of Strictly Come Dancing on BBC1.

Boyzone and Westlife, the Irish boybands, also performed with the other finalists, Eoghan Quigg and JLS, respectively. Quigg went out of the competition after the first count of votes was taken.

Another winner last night was Leonard Cohen, the Canadian singer-songwriter. All three finalists had recorded his song Hallelujah, so Burke’s version, which was her final performance of the night, was being released as a download at midnight and as a CD single tomorrow.

It means a huge payday for the lugubrious Cohen at the end of a year in which he has been forced to tour at the age of 74 after his former manager allegedly bled his bank account dry while he was training to be a Buddhist monk. With the song expected to sell 1m copies in the UK over the next three weeks, it will give him a windfall of at least £250,000, based on a 10% royalty from CD single and download sales.

It is more than the singer of the song will get. Although the winner of The X Factor is guaranteed a £1m record deal, they are given only a £150,000 advance for their first single. The rest goes on recording costs.

Cohen said last week that he was aware of the new version and “marvel[led] at the continued interest in the song”.

Paul Gambaccini, the co-founder of the Guinness Book of Hit Singles, said: “There are worse scenarios than having a Leonard Cohen song at No 1.”

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