Actor Mathew Horne collapsed on stage during a matinee performance of Entertaining Mr Sloane in London today.
Fellow actor Simon Paisley Day called for doctors in the house at Whitehall's Trafalgar Studios as Horne lay prone for 10 minutes before paramedics arrived, reports suggested.
Horne, 30, best known for sitcom Gavin and Stacey, was treated on stage after falling to the floor 15 minutes into the second half of the show. He was taken to hospital but had left by early evening, when he was said to be "feeling better". He was replaced by understudy Fergus March for the evening performance.
His publicist Rupert Fowler said : "Matt has been working flat-out for the last three months. He has patently been suffering from exhaustion." A decision will be taken on when he might return to the Joe Orton play, which ends its run on 11 April.
John Rigby, a BBC producer who was in the audience, said Horne seemed "absloutely fine" in the first half of the play but there was a delay during the interval. The audience seemed to think Horne's collapse was part of the show until Paisley Day called out: "I think something has gone wrong. Is there a doctor in the house?"
An audience member went to help while another called out for someone to dial 999. An actor on stage replied: "Don't worry , it's been done."
Horne had fallen with his back to the audience and did not move except to shake his head while ambulance staff were treating him. Co-star Imelda Staunton apologised for the interruption and said she was "incredibly sorry" for the events. She was interrupted by applause three times. Rigby said: "I think everyone was just trying to show their concern for Matthew's wellbeing."
Horne and James Corden, one of his Gavin and Stacey co-stars, have been extremely busy in recent months. They recently appeared in the film Lesbian Vampire Killers and in BBC3 sketch show Horne and Corden.
Copyright Speakers Corner 2017