Hollywood actors are increasingly endorsing cigarette brands in films, leading to allegations by medical researchers that the movie and tobacco industries might be flouting a ban on payments for "product placements" in films.

The use of actors to promote the most popular tobacco brands has increased tenfold since the voluntary ban was introduced in the US in 1990, according to a report published in the Lancet.

The findings have alarmed anti-smoking groups. Studies have shown that smoking by teenagers is strongly influenced by the use of tobacco by their favourite actor.

"Tobacco companies know full well that kids are going to see these films and this is probably a deliberate tactic to target the youth market," said Amanda Sandford, research manager for the charity Action on Smoking and Health.

The researchers, from Dartmouth Medical School in the US, said that several possibilities could explain continued tobacco brand appearance in films, from the tobacco industry continuing to pay directly or through payments in kind, to directors using brand imagery to increase a sense of realism or to convey character traits. However, flouting the payment ban would be consistent with regular violation of the cigarette advertising code by the tobacco industry since its inception in 1964.

The study looked at the top 25 US box office films for each year from 1988 to 1997. The researchers found that before 1990, 1% of the top US box office films showed actors with a recognisable brand of cigarette on screen. But in the years after the ban, 11% of the films showed actors endorsing a brand.

The four most highly advertised US cigarette brands accounted for the most on-screen appearances, including those in hits such as My Best Friend's Wedding (pictured), Men in Black and Volcano.

More than a quarter of all 250 films studied featured recognisable tobacco brands, including shots of billboards, shop fronts, logos and packets.