Witnesses suggested Lily reacted after being heckled as she left Ronnie Scott's Jazz Club at 2am. With a short reach, the punch appeared to fail to connect.
The Smile singer's friends, including TV presenter Miquita Oliver, then managed to restrain Lily, persuading her to get into a car.
Pow: Pop singer Lily Allen lashes out at a passer-by following a drinking session in Soho in the early hours of this morning
Thwack: The woman tries to protect herself as Lily lets rip with another punch
Biff: With cigarette still in hand Lily displayed her street-fighting skills in front of a battery of paparazzi
Lily's target appeared to laugh off the attack, looking more bemused than terrified.
This isn't the first time the 23-year-old has let her temper get the better of her.
She was arrested and cautioned for assault after kicking a photographer during a night out in Soho last year.
The pop star was leaving the Wardour club when she hit out at the man, in his forties, who was waiting outside.
The joker: Lily's target appeared to laugh off the attack, looking more bemused than terrified
Yesterday Lily and her friends had started their night at the Groucho Club, before heading to Ronnie Scott's Jazz Club.
Earlier in the evening, Lily was in a friendlier mood as she chatted to photographers outside Ronnie Scott's and playfully punched the air.
Lily told the paparazzi that their attention made her feel like a zoo animal and suggested she entertain them with a joke.
Lily, dressed in a purple wrap dress and peep-toe high heels, then managed to expose herself as she climbed into her car.
Where's your bra Lily? The singer exposed herself as she clambered into her vehicle
Lily is clearly quite cross at the moment. Writing on her MySpace blog, she blamed her record label EMI for delaying the release of her new album.
She wrote: 'The album has been finished for a while now, I don't really know what's going on with it.
'Lots of people have been fired or taken redundancy as the company was taken over by a private equity firm. Many of these people were people assigned to my projects.'
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