The Vote - set in a fictional polling station - will make theatrical history when it is shown live on election night, as polls close on May 7 from 8.30pm.
Consisting of 50 actors in total, it will also star Catherine Tate and EastEnders' Timothy West.
The play from James Graham will open at London's Donmar Warehouse for two weeks ahead of the live broadcast. Graham also wrote Channel 4's Nick Clegg drama Coalition.
Dench will play a voter who arrives at the station with her daughter, played by real life daughter Finty Williams.
"This is a unique opportunity for the Donmar to make a small theatre feel very big," said artistic director Josie Rourke.
"Some of the nation's greatest actors are coming together this election night to give
It's Bogof on Halls at the National right now, and the critics have been filling up their trolleys. "This production of Twelfth Night is [Sir Peter Hall's 80th] birthday present to himself and us," explains the Telegraph's Charles Spencer, "and it stars, movingly and magnificently, his daughter Rebecca Hall, now better known as a rising star of Hollywood."
"It catches perfectly the play's melancholy and preoccupation with time, transitoriness and loss," says our own Michael Billington, "even if it cannot efface golden memories of the one he did at Stratford in 1958." (No indeed. 1958, how will any of us forget it?) Even the Hollywood Reporter gets in on things, using words like "players" and "clad" to prove that, yes that's right, it does British theatre. (So long as there's a movie star involved.
The veteran actress starred in the programme - set in a fictional village called Cranford in Cheshire, England - alongside her daughter Finty Williams in 2007 and recently reprised her role for two new Christmas episodes.
And revisiting the set made Dench realise how much she misses the sense of community spirit that she experienced in her youth.
She tells Yours magazine, "It (Cranford) is still the place where people watch out for one another and know each other's business. It takes me back to when I was a little girl in the Thirties and Forties. I remember friends of ours being blitzed (bombed during World War II) in York and everybody rallied round bringing food and clothes, which was wonderful. Now we tend to shy away from that. I'm always rather thrilled that people can't find my house on their Sat Nav (satellite navigation system). But still, there is something to be said for a community that cares."
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