Majorly sad news from the British comedy scene ... Tony Slattery -- best known for his role on the UK version of the improv show 'Whose Line Is It Anyway?" -- has died.

Slattery’s longtime partner of nearly 4 decades, Mark Michael Hutchinson, confirmed the news ... telling BBC the comedian passed away Tuesday morning after suffering a heart attack Sunday evening.

Tony rose to fame in the '80s and was best known for his quick wit and improvisational genius on Britain's hugely successful "Whose Line Is It Anyway?" -- long before the show was remade for U.S. audiences.

His comic career also included roles in "Jeeves and Wooster," the sketch comedy series "A Bit of Fry & Laurie" alongside Stephen Fry and Hugh Laurie, and numerous stage productions.

Tony was most recently touring a comedy show in England and launched a podcast, "Tony Slattery’s Rambling Club."

He was 65.

RIP