British and American tobacco companies deliberately added powerful appetite-suppressing chemicals to cigarettes to attract people worried about their weight, according to internal industry documents dating from 1949 to 1999.

Chemical additives are just one of several strategies successfully used by tobacco companies over the past 50 years to convince people that smoking makes you thin.

Tobacco giants Philip Morris and British American Tobacco added appetite suppressants to cigarettes, according to the documents, released during litigation in the United States.

Four other major companies tested potential chemicals, including amphetamine and nitrous oxide, better known as laughing gas, but the documents, which are incomplete, do not reveal if such chemicals were ever added and sold to the public.

The presence of appetite-suppressing chemicals could help explain why smokers who quit often gain weight, according to Swiss researchers in the European Journal of Public Health.

They call for stricter rules on tobacco additives amid suggestions that sensitive documents are being removed from databases by the industry to avoid disclosure.

Critics say this is further evidence of tobacco companies targeting smokers concerned with weight gain. They argue the industry continues to exploit such anxieties through sophisticated marketing and packaging.

Professor David Hammond, a tobacco industry expert at Waterloo University, Ontario, Canada, said: "We know the industry explored ways to exploit concerns about weight loss back in the 60s, because they knew it was an issue that concerned women, who they wanted to recruit as smokers.

"We don't know if appetite-suppressing molecules are still added, because compliance with additive regulations is poor and sensitive internal documents are usually shredded."

By the end of the 20th century, smoking was mostly male behaviour, but the number of women smokers jumped in the 1940s and 1950s.

A Canadian study of 500 young women, published in Tobacco Control this month, found those looking at female-oriented cigarette packs branded with words such as "slim" and "vogue", were more likely to believe smoking helps people control their appetite compared with those viewing plain packaging.

Smokers wrongly believe certain words, such as the names of colours, and long, slim cigarettes mean the brand is less harmful, says a study published in Addiction. Since the widespread advertising ban, images such as Kate Moss smoking on the catwalk have become invaluable for the industry.

Australia is the first country to introduce compulsory plain packaging. In Britain, the Government committed to considering similar controls on packaging in the Tobacco Control Plan published last month. Public Health Minister Anne Milton will meet her Australian counterpart soon.

A Tobacco Manufacturers' Association spokesman said: "Plain packs are likely to lead to further increases in smuggling ... and make it easier for counterfeiters to copy."