Aviation authorities in Britain have introduced relaxed flight safety rules to minimise more disruptions caused by last month's volcano eruption in Iceland.
The Civil Aviation Authority said it had agreed with airlines, regulators and engine manufacturers on new rules that would let aircraft fly for a limited time through higher ash densities than currently allowed.
The rules, which go into effect at 1100 GMT on 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 only Flybe, the British budget carrier, satisfied those conditions, but it expected other airlines to follow soon and European authorities to introduce similar rules.
The UK's National Air Traffic Services 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,'' Richard Deakin, the company's chief executive, said.
"As a result of this change, there are no predicted restrictions on UK airspace in the immediate future."
London's Heathrow and Gatwick airports and Amsterdam's Schiphol airport reopened on Monday after they closed over the weekend because of worries over volcanic ash which can clog plane engines.
All three warned travellers it would take time for airlines to clear the backlog of delayed flights and advised them to contact their airlines before going to the airport.
The Icelandic civil protection agency said the ash cloud was now drifting to the north.
The April 14 eruption at Iceland's Eyjafjallajokul volcano forced most countries in northern Europe to shut their airspace between April 15 to April 20, grounding more than 100,000 flights and an estimated 10 million travellers worldwide.
The shutdown cost airlines more than $2bn and carriers complained about what they described as arbitrary closures.
Willie Walsh, the chief executive of BA, 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.
Andrew Haines, the Civil Aviation Authority's chief executive, denied that the previous blanket ban on European airspace was an overreaction.
But he acknowledged that making aircraft avoid ash completely was impractical because of Europe's congested airspace.