But the study commissioned by German industrial gases group Linde and carried out by energy consultant e4tech and Imperial College, London, concluded the cost of building 2,800 hydrogen stations across the continent was manageable.

"The results of this study are a clear signal to us," Linde President Wolfgang Reitzle said in a statement. "A transition to the hydrogen economy is feasible."

The study, presented at a Berlin conference, left open who would foot the bill for the network, but a Linde spokesman said energy companies and the automobile industry would have to take the lead.

"It would certainly be welcome if we got support from the political side as well," he said, suggesting governments could encourage a switch to clean fuels by freeing them of tax.

The study assumes some 6.1 million hydrogen cars will be on European roads by 2020. It foresees building up tank stations in heavily populated areas and along motorways first, eventually giving 120 million Europeans - a third of the EU population - access to hydrogen fuel.

The filling stations could support cars with fuel cells, which use the chemical reaction between hydrogen and air to generate electricity and emit only water vapour, as well as those that burn hydrogen in converted standard engines.

Linde also proposed installing some 35 new hydrogen pumps on German highways linking Berlin, Leipzig, Munich, Stuttgart and Cologne to create a giant test track for hydrogen-powered vehicles.

This could help Europe compete with campaigns in California and Japan to develop real-world hydrogen networks, it said.