Britain yesterday ordered a fleet of navy ships to rescue stranded air passengers as airlines stepped up an angry campaign to fly again in the volcanic cloud which has paralysed Europe's airspace.

Nearly seven million passengers have been affected by blanket shutdowns which governments say are essential but which airlines blame for unnecessary chaos and massive financial losses.

Under pressure to assist its nationals, Britain ordered its navy aircraft carrier HMS Ark Royal and HMS Ocean and HMS Albion to pick up thousands of Britons from France - where they have come from all over Europe - and Spain.

"This is the biggest challenge to our aviation transport network for many years," Mr Brown said.

The huge ash cloud kept up its regime of chaos over Europe, forcing the cancellation of another 20,000 flights, though experts said the eruption in Iceland was losing power.

But even as airlines argued the dangers had been exaggerated, a senior US military official said the ash had affected one of Nato's F-16 fighter planes, which detected a glass build-up inside its engine, warning of an impact on military capabilities.

Ash from volcanos can be turned into a glass form at high temperatures when it passes through a jet engine.

"This is a very, very serious matter that in the not too distant future will start having real impact on military capabilities," said the official.

"I think the airspace is closed for a reason."

Europe's three main airports in London, Paris and Frankfurt, remained giant ghost towns as Britain said it would start re-opening its airspace today but experts predicted severe disruptions to run for days.

There were scenes of mayhem at airports across the Asia-Pacific region as thousands of backpackers, business travellers and students anxiously awaited the chance to return to Europe.

In Europe itself, marooned passengers juggled hellish combinations of rail, boat and road links, zig-zagging across borders in desperate attempts to make it home - whether to the other end of Europe or to the US.

Germany also gave its carrier Lufthansa the exceptional go-ahead to repatriate 15,000 passengers on board 50 long-haul flights from around the world.

And Spain, one of the rare countries operating normally, struck an agreement with Britain, France and Germany to fly hundreds of thousands of their nationals back to Europe via Spanish airports.