The 24-year-old former X Factor winner had been meeting members of the public at Waterstone’s Piccadilly in London for around an hour-and-a half on Wednesday afternoon when a man who had been waiting in the queue suddenly lashed out, he said.
Lewis was bruised, upset and shaken after the incident and was checked over by a doctor.
The singer’s spokesman, who witnessed the attack, said: “She was doing a signing to launch her new book. Hundreds of fans had been queuing since last night.”
He said she had been happily signing autographs for around an 90 minutes when “a guy came up and punched her to the side of the head”.
He did not hear the man say anything to Lewis during the “unprovoked” incident and security officers swooped.
Lewis sustained bruising to the side of the head and was left “shaken”, he said, adding that she had gone to see the doctor. “She is absolutely gutted she did not get the signings finished,” he said.
The spokesman said that to hit a woman was “beyond cowardly”.
The former X Factor winner was signing copies of her autobiography, Dreams.
Police said officers were called to Waterstone’s and a man has been arrested “following reports of an assault against a female”.