Downton Abbey star Laura Carmichael has forgiven theatre boss Sir Peter Hall for heckling her during a play after he wrote a grovelling letter of apology.

The director admitted he was mortified after shouting out to Carmichael in 2012 when she was onstage in London's West End in Anton Chekhov's Uncle Vanya.

Hall was heard calling out, "Stop, stop, stop! It doesn't work and you don't work. It is not good enough. I could be at home watching Tv", and the incident sparked major embarrassment for the theatre mogul when he later admitted he had drifted off to sleep and awoke in a confused state.

Now Carmichael insists she has forgiven Hall after he penned her a personal note apologising for his behaviour.

She tells British magazine Event, "Genuinely when I was first told my reaction was: 'I can't believe he came to see my play.' He's a legend. I've read his books and seen his productions. He's old. He fell asleep. But it didn't ruin things, we kept going with the show."

After receiving the written apology from Hall, Carmichael wrote back: "Of course. Don't worry about it. I am so thrilled you were there."