The deals with A-list screen idols of the 1930s and 1940s were worth millions in today's money.

Clark Gable, Spencer Tracey, Joan Crawford, John Wayne, Bette Davis, Betty Grable and singer Al Jolson all appeared in endorsements for brands like Lucky Strike, Old Gold, Chesterfield, and Camel.

Researchers in California claim the continued presence of on-screen smoking in today's mainstream films is rooted in "studio era" deals.

They accessed cigarette endorsement contracts between tobacco companies and studio-controlled movie stars, as well as advertisements of the period.

In return for the paid testimonials of their stars in cigarette advertisements, major studios benefited from nationwide print and radio advertisements for themselves and their movies in lucrative "cross-over" deals, the research shows.

Efforts had been made to outlaw the practice but the archived material shows the studios took advantage of actors' contracts, which gave them complete control over the use of their celebrities.

They were able to negotiate the content of the testimonials, and insist that the endorsement ads, publicising new movies, coincided with their release to cinemas.

In all, almost 200 actors took part in the cigarette endorsements, including two thirds of the top 50 box office Hollywood stars from the late 1930s through to the 1940s.