Press reports of guilt-free aviation from a futuristic aircraft design are overstated, according to the aeroplane's designers.

The A2, a proposed hypersonic airliner based on reusable spacecraft technology, could theoretically travel at Mach 5. It would take 300 passengers from Brussels to Sydney in under five hours, whisking people from Europe over the North Pole and the Pacific and down to Australia.

The craft would emit no carbon dioxide, because it would be powered by liquid hydrogen, which is an effective way to power an aircraft at such high speeds.

The prospect of zero-carbon flights has led several reports to praise the proposed plane's environmental credentials. However, Richard Varvill, technical director of the aircraft's designer, Reaction Engines, downplayed the design's green credentials. Instead he highlighted the environmental problems posed by creating the large quantities of hydrogen fuel that would be required.

Using today's techniques, copious amounts of fossil fuels would be needed to create and transport the 200 tonnes of liquid hydrogen needed for each A2 flight.

Long term, electrolysis using electricity to split water molecules into hydrogen and oxygen might be a viable approach, but could create other hazards. "In this case almost certainly you would need nuclear power – and lots of it, " Varvill said. "You would need a nuclear power station placed near an airport, generating the power, and producing the liquid hydrogen and piping it to the airport."

Varvill, who pointed out that anything shorter than a transatlantic flight for the A2 would not be viable, also raised another concern. The A2 would fly higher than normal aeroplanes, placing it in the middle of the ozone layer, where it would deposit water vapour, the waste product from hydrogen.

"What's not known is whether the release of water vapour into the ozone layer has any deleterious effect on it," he warned.

That is one subject which will be tackled by the four-year LapCat 2 project, an EU-funded initiative following on from the LapCat study which funded the A2's design. Due to start by the end of this year, LapCat will also pursue an alternative Mach 8 aeroplane design that is competing with the A2.

With futuristic ventures still presenting environmental issues, aircraft manufacturers are looking for alternatives. Airbus recently completed a test flight of its A380 passenger aircraft using a mixture of conventional jet fuel and liquified gas, which the firm claims emits almost no sulphur. Meanwhile, Virgin Atlantic will later this month undertake the first test flight of a commercial airliner using biofuel.