A major tobacco company has been accused of deliberately obstructing access to key documents.
BAT is one of the world's leading tobacco companies
Under a 1998 court ruling, British American Tobacco is required to make many of its files publicly available.
But in an article in The Lancet, researchers say they are finding it difficult to access the documents. The company has rejected the claims.
Researchers believe the files may contain secret information on how BAT has tried to boost sales.
They believe the files may also include details of strategies to target children and people in developing countries.
The documents are stored at offices in Guildford in the UK and in Minnesota in the United States.
Researchers say that, while they have good access to files in the US centre, they have struggled to see those stored in the UK.
The US centre is managed by an independent legal firm while the UK centre is managed by BAT.
The researchers say shorter opening hours, restrictions on the number of people who are allowed to visit the centre at any one time and delays in obtaining files are hindering their work.
A team at the London School of Hygiene and Tropical Medicine have now submitted a request to BAT for all of the documents it is supposed to make public.
The company has said it will take 18 months to supply the 40,000 files, which run to an estimated eight million pages.
The researchers say they will publish the papers on the internet when they receive them. Some have already been published.
"These documents are incredibly important for enabling tobacco control policy advocates to understand how the industry has behaved over the past few decades," said Dr Kelley Lee, a researcher at the London School of Hygiene and Tropical Medicine.
"A lot of these documents describe medium term plans, strategies, marketing practices and more dubious strategies which allow them to continue to market and make record profits in the developing and developed world."
Dr Lee accused BAT of playing "a cat and mouse game" with researchers.
"They know we want the documents. They have made it as difficult as possible for us to get them.
"One can only conclude that they don't want us to have them and that they don't want the world to have them."
However, Michael Prideaux, BAT's corporate and regulatory affairs director, dismissed the claims.
"It's important to remember that Guildford was set up as a litigation resource rather than a public library.
"Although it was subsequently opened to the public following the settlement of litigation in Minnesota, it was never designed to work like a public library.
"The researchers' ability to publish so many pages shows that they can't have found access that difficult."
He added: "I doubt that this collection of old documents will contribute much fresh insight to the subject of smoking and health.
"If there was a 'smoking gun', presumably someone would have found it by now."