The trial, which ran on flights between Frankfurt and Hamburg, will end January 12 on a flight from Frankfurt to Washington.

"Lufthansa will only continue the practical trial if we are able to secure the volume of sustainable, certified raw materials required in order to maintain routine operations," project manager Joachim Buse said on Monday.

The race to cut carbon dioxide emissions has heated up with the introduction this month of the European Union emissions trading scheme under which airlines must pay for the CO2 they emit.

Lufthansa has said previously the scheme will lead to higher ticket prices, and major U.S. airlines such as American Airlines (AAMRQ.PK) and Delta (DAL.N) have added surcharges.

While the airline industry wants to use biofuels to curb emissions, the lack of industrial production means only limited quantities are available.

European airlines, biofuel producers and the EU Commission last year signed a pact aiming to produce 2 million tonnes of biofuel for aviation by 2020.

The environmental benefits of some biofuel mixes have also been called into question as some crops such as palm oil use land that could instead be used to grow crops to feed people.

Other non-plant-based options for creating biofuel include waste and algae. British Airways (ICAG.L) is hoping to start powering its fleet using a fuel derived from waste by 2015, while UK rival Virgin Atlantic plans to start using a waste gases-derived fuel by 2014.

Lufthansa's Buse said the six-month trial between Frankfurt and Hamburg, which saw one engine of an Airbus A321 powered by a 50:50 blend of regular fuel and biofuel, had been a success.

The airline operated 1,187 biofuel flights, with initial calculations showing CO2 emissions were reduced by 1,471 tonnes.

It said the flight from Frankfurt to Washington would cut emissions by 38 tonnes, equivalent to the CO2 emissions of six scheduled flights between Frankfurt and Berlin.

(Reporting by Victoria Bryan; Editing by David Cowell)

• A Permanent Economic Emergency