Two of America's richest men have teamed up to fight big tobacco.

Bill Gates and Michael Bloomberg have launched a fund to help poorer countries battle legal challenges from tobacco firms.

The $4 million fund is backed by Bloomberg Philanthropies and the Bill & Melinda Gates Foundation. The Microsoft (MSFT) co-founder said developing countries don't have the resources to fight tobacco firms in the courts.

"Country leaders who are trying to protect their citizens from the harms of tobacco should not be deterred by threats of costly legal challenges from huge tobacco companies," said Gates.

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It comes as the industry continues to mount challenges to anti-tobacco legislation around the world.

The Bloomberg-Gates alliance said several countries have been threatened by the industry, and that has hindered governments from implementing tobacco control laws such as graphic health warnings and branding bans.

More litigation is likely. British American Tobacco (BTI) said it will launch a legal challenge against the U.K. government following its plan to ban branding on cigarettes.

"Plain packaging" legislation is designed to reduce smoking rates by dulling the appeal of tobacco products and has been used in Australia since 2012. The effectiveness of the laws have been widely contested.

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The new fund is part of a wider push by the billionaires to stamp out tobacco use around the world. Bloomberg Philanthropies has committed $600 million since 2007 to fight tobacco use, and the group has been helping Uruguay fight Philip Morris (PM) in the courts over the country's use of graphic health warnings on cigarettes.

Tobacco will claim one billion lives this century unless urgent action is taken, the philanthropists say.