Conservative billionaire industrialist David Koch is retiring from his roles at Koch Industries and associated companies due to poor health, according to an internal memo.

David Koch, 78, along with his elder brother Charles, have been a force in American politics since the 1980s.

Their influence has largely been powered by a fortune centred on Koch Industries, the second-largest privately held company in the United States, whose operations range from refining and chemicals to ranching and forest products.

The brothers are known to spend heavily on conservative initiatives and to oppose government intervention in business.

David, who was diagnosed with prostate cancer almost two decades ago, is the executive vice-president at Koch Industries.

The brothers have donated several millions to doctors and researchers working towards a cancer cure.

"David has always been a fighter and is dealing with this challenge in the same way," Charles Koch wrote in the memo dated June 4.

The company said no decision had been made on David Koch's replacement.

David will also step down from his role as chairman for Americans for Prosperity, a grassroots conservative group that is a part of the Koch network, the memo said.

Brothers face off with Trump

The news comes a day after the brothers said they would launch a multi-million-dollar campaign to oppose President Donald Trump's tariffs, and to champion free trade.

The Kochs, who have channelled billions of dollars into conservative causes, kept their distance from Mr Trump during the 2016 presidential election.

Charles Koch spoke out against Mr Trump's proposed Muslim registry.

The brothers also strongly support legislation that seeks to protect "Dreamers" — people brought illegally to the United States as children — from deportation.

However, the Kochs found common ground with the new Republican President on issues including rolling back federal regulations and pulling out of the Paris global climate accord, after the election.

Koch Industries was founded by Fred Koch in 1940, and was then called Wood River Oil and Refining Company.

Charles Koch, Fred's first-born, joined the company in 1961, and in 1967, renamed the company Koch Industries Inc to honour his father.

Reuters