By the BBC's Corinne Podger

Tobacco companies may be failing to abide by a voluntary agreement to curb indirect advertising in films, according to researchers at the University of California.

Smoking is just as prevalent in Hollywood films as it was before the agreement was reached in 1990, according to the findings published in the British Medical Journal.

The paper's authors trawled through more than 1,500 tobacco industry documents, which detailed public relations efforts with the Hollywood entertainment industry up to the mid-1990s.

Many of the documents were previously secret industry publications, released into the public domain as a result of litigation against tobacco firms.

They date from both before and after a series of Congressional hearings on product placement in 1989, which led to a voluntary agreement to curb the placement of tobacco products in films.

'Strong positive images'

The researchers found that tobacco firms explicitly referred to the "strong, positive images" that were "created by cinema and films" for cigarettes and smoking.

The documents also discuss payments made to actors and directors who agreed to place tobacco products in films, or show actors smoking on-screen.

The last of the documents dated from the mid-1990s. But the authors claim that smoking has been shown more, rather than less, often throughout the 1990s.

By last year, cigarettes and smoking were shown on-screen more often than in the 1960s.

One of the tobacco firms mentioned in the study is Philip Morris.

In a statement, the company denied the suggestion that it was failing to abide by the voluntary ban on indirect advertising, and said it did not pay for product placement in films or television shows.