The actor Alex Beckett - who played Barney Lumsden from Perfect Curve in BBC comedy W1A - has died suddenly, his agent has confirmed.
Beckett, 35, was also known for his theatre work and had been starring in The Way of the World at the Donmar Warehouse since last month.
The rest of this week's shows have been cancelled as a mark of respect.
He was described by acting agent Gavin Denton-Jones as "a wonderful man and a hugely talented actor".
He added: "Our thoughts are with his family and we kindly ask that their privacy be respected at this difficult time."
The Welsh actor had been playing Waitwell in the restoration comedy by William Congreve, first performed in 1700.
It opened to positive reviews last week, having been in previews since 29 March. It is due to run until 26 May.
Shane Allen, controller of comedy commissioning at the BBC, said: "We're all incredibly crushed to hear of Alex's untimely death.
"He was a very prolific, versatile and much admired comedy star whose role as Barney Lumsden in both Twenty Twelve and W1A was a key ingredient of their success.
"We think of him fondly and our hearts go out to his family and friends at this painfully sad time."
In W1A and Twenty Twelve, Beckett's character was part of the quirky brand consultancy agency Perfect Curve, led by Jessica Hynes, who played PR Siobhan Sharpe.
Hynes said she was "so sad he is gone".
Alex Beckett was a wonderful, clever, kind, brilliant person - so glad to have known him, so sad he is gone.— Jessica Hynes (@JefficaHoons) April 12, 2018
Fellow W1A cast member Rufus Jones, who played David Wilkes in the show, said Beckett was "such a warm-hearted and funny, funny guy".
Just hearing this awful news about Alex Beckett. He was such a warm-hearted and funny, funny guy. W1A felt like a big family and he was a face you couldn’t wait to see. My thoughts with his family xx— rufus jones (@rufusjones1) April 12, 2018
John Morton, the writer, director and creator of W1A and Twenty Twelve described Beckett as "always so funny, so full of life, so full of generosity, a pleasure to see every day".
He added: "All of us involved in W1A and in Twenty Twelve before that were lucky to know him, lucky to work with him, and his loss is desperately sad."
Josie Rourke and Kate Pakenham of the Donmar Warehouse added: "We are deeply shocked by the tragic loss of our dear friend and brilliant actor Alex Beckett, a much-loved member of The Way of the World company.
"Our thoughts are with his family and close friends at this incredibly sad time.
"Everyone at the Donmar and The Way of the World company are completely devastated by this terribly sad news."
The theatre said this week's shows had been cancelled to "give some time to the company, who all loved Alex as a fellow actor, and a friend".
The Orange Tree theatre, where Beckett had performed in 2016, said he would be "much missed".
All at the Orange Tree are shocked and saddened by the sudden death of Alex Beckett. A superb actor of great range, charm, intelligence and wit, he shone in Caryl Churchill’s Blue Heart at the OT in 2016. An enormously warm, kind man: he will be much missed. pic.twitter.com/P8vULIc6uJ— Orange Tree Theatre (@OrangeTreeThtr) April 12, 2018
Beckett was born in Carmarthenshire in 1982. His theatre credits include Grimly Handsome at the Royal Court, Headlong's Pygmalion and Edward II at the National Theatre.
Both the National and Jeremy Herrin, Headlong's artistic director, have paid tribute on Twitter.
Sad to hear of the sudden death of actor Alex Beckett. His most recent work with us was in Edward II in 2013. Our thoughts are with everyone at the @DonmarWarehouse, where he was performing in The Way of the World. Here is their statement: https://t.co/yMHl5T1bIi pic.twitter.com/sb2olxEqWN— National Theatre (@NationalTheatre) April 12, 2018
Heartfelt condolences to Alex Beckett’s family. We are shocked and devastated at Headlong. He was such a vital Higgins in our production of Pygmalion: it’s very difficult to imagine him gone.— Jeremy Herrin (@JerHerrin) April 12, 2018
Beckett was also remembered by Ash Atalla, producer of The Office, who recently worked with the actor on E4 sitcom Stath Lets Flats.
My god Alex Beckett has died. A lovely and very Welsh bloke. He’s been in a few of my shows, one that just finished filming. Huge love to his family.— Ash Atalla (@AshAtalla) April 12, 2018