Britain's armed forces could acquire a new tinge of green under plans to end the military dependency on fossil fuels.

Possible innovations include unmanned attack aircraft powered by the sun. They would fire missiles fuelled with hydrogen produced by feeding algae to microbes.

Tanks could be electrically powered or run on fuel produced from oil squeezed out of weeds so hardy they can grow in the desert.

Ships could run completely on electricity produced from generators powered by synthetic fuels made from grass.

The environmental requirements of the army, navy and air force will be presented this week to specially vetted defence and research companies.

The Ministry of Defence (MoD) said last week that many of the ideas would come to fruition only in the next generation. The US Air Force, however, is expected to start converting its aircraft to use a mix of synthetic and petroleum-based fuel by the end of 2010 and the RAF is likely to follow suit.

The Royal Navy’s new Type45 destroyers already use all-electric propulsion, albeit produced by gas generators, and greener ways of producing the electricity are being explored in conjunction with the French.

Officials said last week that the need to reduce the carbon footprint of the forces’ gas-guzzling tanks, jets and other equipment was just one factor: green fuel is also seen as an opportunity to cut the forces’ £400m annual fuel bill, which has doubled in four years.

It is also hoped that diversifying fuel supplies will reduce reliance on the unstable Middle East and cut transport costs.

Turning all three services “green” was one of a number of new defence targets outlined last week by Paul Stein, the MoD’s science and technology director, to the Royal Aeronautical Society in London. Others included more unmanned, missile-carrying aircraft; lightweight tanks; ultra-light equipment for soldiers and futuristic communications devices.

While some of the plan will be made public, most will be made available on a secure website to selected company and university research departments.

“It will provide clear direction to the research and development community,” the ministry said.

The MoD’s science and technology experts envisage more efficient engines and greater use of solar power, microbe-powered fuel cells and lightweight and remotely operated aircraft and robots.

Future biofuels are likely to focus on inedible plants such as the jatropha, which thrives as a weed on arid land and desert and needs little water. It is already being cultivated in dry parts of India for biodiesel.

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