Gavin & Stacey star Matthew Horne has been replaced by an understudy after collapsing onstage during today’s midweek matinee performance of Joe Orton’s Entertaining Mr Sloane.

Matt Horne “seemed fine” during Act I, but audience members suspected something was wrong after the interval was unexpectedly stretched out. During a scene between Sloane (Horne) and Ed (Simon Paisley Day), Horne is described as either having “collapsed” or “fainted”, leaving Paisley Day stranded. Paisley Day had to appeal to the audience for help, asking any doctors in the audience to come forwards:”I think something’s gone wrong, is there a doctor in the house?“.

One audience member immediately moved to the front of the stage (BBC) while another offered to call 999, to be told it had been done. Audience sources say stagehands also came onto the stage, one speaking to reassure the audience.

Horne lay prone and apparently unconscious on the stage for ten minutes before paramedics arrived; he was treated onstage before a stage manager stepped forward asked audiences to leave the theatre. Horne did not move or speak during treatment, except to shake his head once when addressed by paramedics.

This evening, publicist Rupert Fowler told Sky News Online that Horne was suffering from “exhaustion”, following eight performances a week as Sloane. Despite this, Fowler was unable to say if Horne had left hospital, how serious his condition was, or if he would be returning to the production.

After Horne’s collapse, actress Imelda Staunton (playing Kath) also addressed the audience, apologising for the interruption, and saying that she had earlier felt ill as well. Staunton’s speech, which promised theatregoers they would be refunded, was on three occasions interrupted by emotional applause from the audience.

The theatre has confirmed that Staunton too will be replaced by an understudy at tonight’s performance. Horne’s understudy has been identified as Dead Wood actor Fergus Marsh.

Horne was using his Twitter account until this afternoon, joking: “What a beautiful day!!! … For doing a matinée and evening performance! Hope all’s tweetness and light for the Team”.

Poor guy. It’s not been a very good few months for West End stars being invalided out, has it? First Tennant, then Dench, now Horne and Staunton…

ETA: 3/4/2009 James Corden’s negative reports on Matt Horne’s condition are untrue; Matthew Horne has been released from hospital; Rob Brydon has unmasked the Twitter “James Corden” as a fake.