British actor and comedian John Sessions has passed away at age 67.

Sessions, known for his appearances on QI and the U.K. version of Whose Line Is It Anyway?, as well as for providing celebrity voices on Spitting Image, died after suffering a heart attack.

His agent Alec Irwin told the BBC: "It is with great sadness we can confirm that on Monday 2nd November, the actor John Sessions died at his home in South London. He will be hugely missed."

During a near-40-year film and TV career, Sessions took on supporting roles in movies including Gangs of New York, The Iron Lady, The Last Station, Florence Foster Jenkins, and Mr. Holmes.

In the 2014 Doctor Who episode "Mummy on the Orient Express," he provided the voice of Gus, a sentient computer controlling the titular train. He also co-created and starred in the cult '90s comedy series Stella Street and made TV appearances in Skins, Outlander, and Friday Night Dinner.

His impeccable impression of his late friend Alan Rickman, which you can watch here, proved especially popular over the years.

Sessions was gay; in a 2017 interview with The Guardian, he spoke about his large extended family and many godchildren, saying:"I now have four great-nieces, three great-nephews, three goddaughters and five godsons, (the youngest of whom is Ian Hislop’s boy, Will, who is doing very well as a standup comedian)."

Actor Samuel West is among the many paying tribute to Sessions. "Hate that John Sessions has gone. World so much better with him in it," he wrote on Twitter. "Lucky to work with him several times. Brain the size of Swindon. So full of life and stories and contradictions and possibilities and love for the job of acting."

The QI team also paid a heartfelt tribute, tweeting: "John Sessions was a panelist on QI's first ever episode: Series A, Episode One, 'Adam'. His incredible wit and encyclopaedic knowledge played a huge part in the show's history and everyone at QI is deeply saddened to learn of his passing."

Rest in Peace, John Sessions, and thank you for your contribution.