A cloud of volcanic ash which has caused chaos for air travellers spread to Germany today, forcing the closure of Berlin's airport and threatening to disrupt European flights for days.

The cloud forced the cancellation of hundreds of flights over Britain yesterday as winds blew the ash from Iceland's Grimsvotn volcano over Scotland, but UK authorities said high-level densities of ash had cleared their airspace today and there were only scattered cancellations, mostly related to Germany-bound flights.

German air traffic controllers said today that they would halt all flights to and from Berlin's Tegel and Schoenefeld airports, starting at 11am (0900GMT). Airports in Bremen, Hamburg and Luebeck have already been closed for hours, grounding hundreds of flights.

While experts say particles in the ash could stall jet engines and sandblast planes' windows, many argue that flight bans are a massive overreaction by badly prepared safety regulators.

A British Airways test flight passing through the affected area was unaffected, said Willie Walsh, the chief executive of International Airlines Group - formed from the merger of BA and Iberia.

"We flew in the red zone for about 45 minutes at different altitudes over Scotland" and the North of England, he told BBC radio. "All the filters were removed and will be sent to a laboratory for testing. The simple answer is that we found nothing."

Irish budget airline Ryanair also challenged the ban, saying yesterday that it had sent its own plane into Scottish airspace and found no ash in the atmosphere.

But German transport minister Peter Ramsauer insisted the precautions were justified, and said authorities were better prepared after the Eyjafjallajokull volcano eruption last year forced the closure of European air space for five days, stranding millions.

"We have developed a very refined regulation since the big ash cloud last April," he told public broadcaster ARD. "We are much better prepared to handle such a situation."

Last year, European aviation authorities closed vast swathes of European airspace as soon as they detected the presence of even a small amount of volcanic ash in the atmosphere. This year they are trying a more sophisticated approach.

Aviation authorities will give airlines information detailed information about the location and density of ash clouds. Any airline which wants to fly through the ash cloud can do so, if it can convince its own national aviation regulators it is safe.

Officials in Iceland have said the amount of ash being released by the volcano is decreasing, and do not expect the disruption to be as bad as last year. The Grimsvotn volcano began erupting on Saturday, sending clouds of ash high into the air.

The main international body representing carriers, the International Air Transport Association, complained to the British Government yesterday about the way it had handled the issue, saying it should have had Cessna planes ready to carry out tests, instead of relying on the weather service.