Aviation authorities introduced relaxed flight safety rules Monday to minimize more disruptions caused by a volcano eruption in Iceland, as three of Europe's busiest airports reopened after a dense volcanic ash cloud dissipated.

Britain's Civil Aviation Authority said it agreed with airlines, regulators and engine manufacturers on new rules that would let planes fly for a limited time at higher ash densities than currently allowed. The rules — which go into effect Tuesday — are subject to airlines getting a guarantee from their engine makers that their aircraft can safely tolerate the ash.

The body said that so far British budget carrier Flybe was the only airline that satisfied those conditions, but it expected other airlines to follow soon and European authorities to introduce similar rules.

British air traffic control company NATS said the new rules meant that restrictions on British airspace could now be eased.

"There is mounting evidence that aircraft can fly safely through areas of medium density, provided some additional precautions are taken. This is now what has been agreed," the company's CEO Richard Deakin said. "As a result of this change, there are no predicted restrictions on U.K. airspace in the immediate future."

London's Heathrow and Gatwick airports and Amsterdam's Schiphol Airport— some of Europe's biggest air travel hubs — reopened Monday after they closed because of volcanic ash worries. All three warned travelers it would take time for airlines to clear the backlog of delayed flights and to contact their airlines before going to the airport.

All British, Scottish and Irish airspace will remain open at least until early Tuesday, but airspace over the North Sea was still restricted, affecting some helicopter operations.

Eurocontrol, the continent's air traffic control agency, said 28,000 flights were expected Monday in Europe — about 1,000 less than normal — mainly due to the disruptions in Britain and the Netherlands.

Iceland's Reykjavik airport was closed Monday. The Icelandic civil protection agency said the ash cloud was drifting to the north, and was not expected to travel to Europe in the next two days.

Germany sent up two test flights Sunday to measure the ash cloud, but there was no word yet on the results of those tests. Still, Germany said Monday the latest ash cloud should not affect its airports.

"At this time, the concentration of ash above German air space is so low that there are no reductions in air traffic," German Air Traffic Controllers said.

Ash can clog jet engines. The April 14 eruption at Iceland's Eyjafjallajokul volcano forced most countries in northern Europe to shut their airspace between April 15-20, grounding more than 100,000 flights and an estimated 10 million travelers worldwide.

The shutdown cost airlines more than $2 billion, and carriers complained about what they described as arbitrary closures. British Airways chief executive Willie Walsh called the latest disruptions "a gross overreaction to a very minor risk."

"I am very concerned that we have decisions on opening and closing of airports based on a theoretical model," he said. "There was no evidence of ash in the skies over London today yet Heathrow was closed."

Last week, the European air safety agency proposed drastically narrowing the continent's no-fly zone because of volcanic ash to one similar to that used in the U.S. The proposal still must be approved.

Eurostar added four extra trains Monday — an additional 3,500 seats — between London and Paris to help travelers cope.

Eyjafjallajokul (pronounced ay-yah-FYAH-lah-yer-kuhl) erupted in April for the first time in nearly two centuries. During its last eruption, starting in 1821, its emissions rumbled on for two years.

———

Associated Press writers Jennifer Quinn in London, Slobodan Lekic in Brussels, Jan Olsen in Copenhagen, Melissa Eddy in Berlin and Mike Corder in The Hague, Netherlands, contributed to this report.