Gavin and Stacey star Mathew Horne has collapsed on stage during a performance of Entertaining Mr Sloane.
The 30-year-old actor was said to have fallen to the floor 15 minutes into the second half of the show during a scene with actor Simon Paisley Day.
Paisley Day called out for any doctors in the house to come forward, with Horne on the stage for 10 minutes before paramedics arrived, the BBC reported.
He was treated on stage before the manager asked the audience to leave the Trafalgar Theatre in London.
Co-star Imelda Staunton was also said to have walked on to the stage and apologised to the audience for the interruption.
A spokeswoman for the theatre said she was trying to establish what had happened and said the show would go on with an understudy if Horne was too unwell.
Horne’s publicist was not immediately available for comment.
John Rigby, a BBC producer in the audience, told the BBC that Horne seemed ‘absolutely fine’ during the first half of the Joe Orton play but that there was a delay after the interval.
It was announced the play would resume in three minutes, but a further 10 minutes elapsed before the actors took to the stage.
He said that the audience seemed to believe that Horne’s collapse was part of the show until fellow actor Paisley Day called out: “I think something has gone wrong, is there a doctor in the house?”