The Robin Hood star came to blows with his soundman, according to Harry Hill's TV Burp writer David Quantick.
He wrote on Twitter: "Keith Allen punched in the face by his soundman at Laugharne."
Allen's daughter Lily then added: "Is this true? Is he okay? I can't get through to his mobile."
Quantick later quoted the soundman's apology to the 56-year-old actor, saying: "I wouldn't have done it if I'd known he was Lily Allen's dad."