As Roj Blake in the cult series, the Welsh actor led a group of rebels on their Liberator space ship against an evil federation that ruled the galaxy.
Born in 1945, Thomas attended the Royal Academy of Dramatic Arts in the 1960s and later became a Rada associate. Yet he remains best known for Blake's 7, which ran on BBC One from 1978 to 1981. At its peak, the series was watched by 10 million viewers and was sold to 40 countries.
Thomas claimed never to have watched a single episode of the show, which was derided by some for its shaky sets and basic special effects.
The show also had a distinctly pessimistic tone - typified by the final episode, in which all the main characters were apparently killed off.
His death was confirmed by a friend who worked on a Blake's 7 remake, according to the Press Association.