The Tokyo-based firm, one of the world's biggest cigarette companies, with brands including Winston and Benson & Hedges, said the planned changes from Thailand's public health ministry would interfere with its operations in the kingdom.
Thailand has decided to extend health warnings from 50 to 85 percent on both sides of every cigarette packet sold in the country.
The new rule is to come into effect in October, but Japan Tobacco said the change would have a "disproportionate impact on legitimate competition, intellectual property rights and freedom of expression".
The company added that adult smokers should be "appropriately informed" before deciding to smoke and smokers "should continue to be reminded" about the health risks.
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"JTI however does not believe that increasing the size of graphic health warnings to cover 85 per cent of cigarette packaging is effective or proportionate," it added.
Philip Morris, which makes the Marlboro brand, and hundreds of Thai retailers yesterday warned they would sue Thai health authorities and claimed the industry was not consulted on the changes from Thailand's health ministry.
Philip Morris has fought bitter legal battles with governments before, most famously losing an action against a pioneering Australian government policy to introduce entirely plain cigarette packaging with the same typeface and graphic images of diseased smokers.
Thailand's Deputy Health Minister Cholnan Srikaew dismissed the threat from Philip Morris.
Thailand bans smoking in public places but figures from its Office of Tobacco Control said smoking rates among those 15 years and older remained roughly unchanged from 27.2 percent in 2009 to 26.9 in 2011.
The tobacco lobby has systematically tried to block laws curbing their ability to advertise their products or raise taxes on cigarettes, but more and more countries are adopting the approach as the health costs of smoking mount.