The Luther actor has been frequently linked to the project, which is scheduled to begin filming in May after more than a decade in the pipeline.
"We are delighted finally to be in pre-production on Long Walk to Freedom," said producer Anant Singh. "It has taken many years to find the right actor, however Idris is perfectly cast to play Nelson Mandela - he has the passion, the star power, the African upbringing and, of course, the height."
Singh, who is producing the film with David Thompson, was entrusted with the rights to Mandela's story in 1997 by the former South African president himself.
Charting the whole of Mandela's life, The Long Walk To Freedom will include scenes shot on Robben Island, where Mandela spent 18 of his 27 years in prison.
The film will be directed by The Other Boleyn Girl's Justin Chadwick and written by Gladiator scribe Bill Nicholson.
It was reported last week that Naomie Harris is poised to portray Winnie Mandela.