He was performing to a crowd while sitting on a stool and suddenly fell silent for a few minutes. As the BBC reports, people in the crowd continued to laugh, thinking it was part of the comedy routine.
Paramedics later arrived and pronounced him dead on the scene.
The show organizer, Andrew Bird, said Cognito had not been feeling well prior to his set but insisted on performing.
"He was like his old self, his voice was loud. I was thinking 'he's having such a good gig,'" Bird told the BBC.
He also noted that Cognito had joked about his own health at the start of the set, quipping, "Imagine if I died in front of you lot here."
Cognito (born Paul Barbieri) was born in London in 1958 and had been performing comedy since the mid-'80s.
See a number of tributes from fellow U.K. comics.
Veteran stand-up comedian Ian Cognito has died on-stage - literally. The audience thought it was part of the act. Died with his boots on. That's commitment to comedy. I'll never forget his kindness when I started out & how god damn funny he was.— Jimmy Carr (@jimmycarr) April 12, 2019
Ian Cognito has passes away. Gigged with him when i first started out and he was always so much fun, had his own mythology on the circuit his exploits where legendary. A true maverick. Hope he's found somewhere to hang his coat in heaven.— Jack Whitehall (@jackwhitehall) April 12, 2019
In shock at the news of Ian Cognito's sudden passing. He was always kind to me when I started out, and brilliant and provocative and entirely original onstage. What a loss.— Matt Lucas (@RealMattLucas) April 12, 2019
Sad to hear about Ian Cognito. I hung with him a good few times in the past, and he was always fascinating and hilarious company. RIP Cogs— Katy Brand (@KatyFBrand) April 12, 2019
Oh bless that Ian Cognito, who expired in his natural home last night, on stage. He was a difficult awkward hilarious troubled brilliant sort; a proper comic.— Mark Steel (@mrmarksteel) April 12, 2019
Ian Cognito was one of the real ones. Always stood up for the underdog. We did Cape Town Fest in 2003, I had an issue with a bigger comic, so he walked across the stage behind him, completely naked. A real force of nature, a true legend and a mate.— Paul Chowdhry (@paulchowdhry) April 12, 2019
Just heard about Ian Cognito. Known him 25 years.— Mitch Benn 🇪🇺 (@MitchBenn) April 12, 2019
Saw him a month ago; looked better than I'd seen him in years. He was 18 months sober and in treatment.
This feels so unfair.
He'd never forgive me if I didn't swear at this so WHAT YOU FUCKING DO THAT FOR, COGS YOU CUNT