The Judge John Deed actor began "mumbling" his lines before passing out during a performance of The Country Girl at Shrewsbury's Severn Theatre last night, claims The Sun.
The 65-year-old apparently whispered "I'm sorry, I'm sorry" as co-star Jenny Seagrove ran to his aid and the curtain dropped. Paramedics from St John's Ambulance shortly arrived on the scene.
An audience member said: "One minute he was acting, then he was a mumbling mess. After a 20-minute interval he was replaced by his understudy."
Shaw's agent blamed the incident on previously-known health complications and the fact that the actor was under medication.
His agent said: "He passed out because he has a cracked rib, a chest infection and is on antibiotics. He probably shouldn't have been on stage but didn't want to disappoint his fans."
As well as playing the title characters in both Judge John Deed and Inspector George Gently, Shaw has also appeared in Coronation Street, Cranford and Agatha Christie's Poirot.