Negga, whose other TV credits include Criminal Justice and Five Daughters, will star in the 60-minute film that explores the life of one of Britain's most popular singers.
"I'm thrilled to be cast in the role of Shirley Bassey and it's an absolute honour to be playing her in such an intimate story of her life," said Negga.
Lesley Sharp (The Shadow Line) will play Bassey's mother Eliza and Charlie Creed-Miles (Injustice) is her manager Mike Sullivan, while Henry Lloyd-Hughes (The Inbetweeners) is cast as Kenneth Hume, her future husband.
"Dame Shirley Bassey is iconic," said executive producer Maxine Watson. "She transcends age, race and class. The big voice and vocal style are unmistakable, but it took so much more than that to take her from Tiger Bay to world class headliner.
"Hers is a classic story of overcoming poverty through personal sacrifice and I'm thrilled that we have such a fantastic cast to bring this story to life."
Shirley Bassey: A Very British Diva is part of BBC Two's Mixed Race season this autumn.
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