Roberts has also agreed to act as chairman of Earth Biofuels’ advisory board.

“It's very important that we expand our use of clean energy and make a long-term commitment to it,” Roberts said on Tuesday.

“Biodiesel and ethanol are better for the environment and for the air we breathe.”

Improving air quality in the US appears to be a major concern for Roberts, whose first task as chair of the advisory board will be collaborating on an “awareness program” to encourage the use of biodiesel in US school buses.

Earth Biofuels estimates there are more than 500,000 diesel school buses currently in service throughout the US.

“The use of biodiesel is a positive step toward minimising pollutive emissions and greenhouse gases,” Roberts said.

“By focusing on school buses, we can affect the health and wellbeing of the people most susceptible to that pollution – our children – today.”

Earth Biofuels appears to have adopted a serious branding approach to building its domestically driven biofuels market: Roberts taking the lead on air quality issues; Willie Nelson’s obvious appeal to the country’s agricultural producers and trucking industry.

Acclaimed actor Morgan Freeman was officially appointed to an executive position on the company’s board in September 2005, at which time he made a statement pre-empting President Bush’s State of the Union address.

“I firmly believe that alternative fuel supplies need to be developed to allow the United States to wean itself off of its significant dependence on foreign oil,” Freeman said.

“Moreover, I feel that our development of alternative sources such as biodiesel fuel will help the environment, farmers, and the economy in general. I look forward to participating in the growth of this company as well as the alternative fuels industry.”

Coincidentally, Freeman was the first African-American actor to play a US president in a major Hollywood production, Deep Impact (1998).

Earth Biofuels appears to have bolstered its credentials as a patriot in the fight for energy independence with last month’s appointment of Herbert Meyer, a former special assistant to the director of the US Central Intelligence Office during the Reagan administration.

“It is with a great sense of purpose that I accept this seat on the Earth Biofuels board of directors,” Meyer said in June.

“I am of the opinion that agriculture-based fuels, such as biodiesel and ethanol, hold tremendous promise for reducing America's dependence on foreign oil, specifically from those regions which are unfriendly and/or hostile to US interests at home and abroad.”

Earth Biofuels chairman and chief executive Dennis McLaughlin said Meyer’s years of patriotic service were a valuable resource for the biofuels company.

“To the degree that United States energy policy becomes an issue of national security, we could not be more pleased to have someone with Mr Meyer's experience and expertise serving on the board,” McLaughlin said.