A small number of flights have taken off in northern Europe after five days of inactivity caused by the spread of volcanic ash from Iceland.But UK air traffic officials said a new ash cloud spreading from Iceland cast doubt on plans to reopen UK airspace.
The UK's air traffic control authority, Nats, says it is unlikely that the main airports in London will reopen on Tuesday, because of more ash in the atmosphere.
Flights have taken off from Scotland and there is limited airspace over the north of England. British Airways says it has cancelled all short-haul flights.
A Nats statement said the situation remained "dynamic" and that "the latest information from the Met Office shows that the situation today will continue to be variable".
Three zones
In an effort to take control of the situation, EU transport ministers created a core no-fly area, a limited-service zone and an open-skies area.
The first flights left Amsterdam's Schipol airport late on Monday. Dutch Transport Minister Camiel Eurlings said his country was "taking a lead" in restarting flights, but warned that further suspensions might prove necessary if the situation worsened.
Swiss and northern Italian airspace has reopened, and limited flights are resuming out of the French capital.
The first commercial flight out of Paris' Charles de Gaulle airport has left for New York. Others are scheduled for San Francisco, Algiers and elsewhere.
The skies over Germany are due to remain closed until 1200 GMT, with some exceptions.
The German carrier, Lufthansa, says it is planning about 200 flights on Tuesday, taking advantage of special permission to fly visually rather than relying on instruments and keeping in constant touch with air traffic controllers.
The BBC's Nik Gowing, at Frankfurt airport, says flights have been arriving from America and elsewhere, and that passengers reported a normal service on board.
But he says that out of 100 flights scheduled in the next 12 hours, some 95 have already been cancelled and he says the situation remains very fluid.
In Madrid, our correspondent Sarah Rainsford says passengers have been arriving on time for their flights but have had to queue for several hours at the information desk to find alternative transport if their flights have been cancelled.
She says that on Monday dozens of coaches were laid on, taking people to Paris, Brussels, and Geneva - and this will be the case again on Tuesday if European airspace does not open up as expected.
The International Air Transport Association earlier labelled the chaos a mess and an embarrassment for Europe.
The body says its losses have soared to over $1bn (£650m; 740m euros), since much of Europe's airspace was closed last week because of ash from southern Iceland's Eyjafjallajoekull volcano.
The European Union has admitted there were shortcomings in the way the decision to close parts of Europe's airspace was reached.
An official with the EU Transport Commission, Helen Kearns, said there was loose co-ordination in the decision-making process and that, working with the same scientific information, different countries had reached different conclusions.
She said, however, that in the future when deciding whether planes should be grounded, there would be no change in the policy of putting safety first.
Warships deployed
Experts had earlier said the volcano - which erupted last Wednesday for the second time in a month - was now spewing more steam and less ash.
In the high temperatures of an engine turbine, ash can turn to molten glass and cripple the engine.
The shroud of fine mineral dust particles from the volcano has spread from the Arctic Circle in the north to the French Mediterranean coast in the south, and from Spain into Russia.
Meanwhile, the UK deployed three Royal Navy warships to help pick up stranded passengers from Spain and the Channel ports.
In Spain, where all airports were open, the government offered to let Britain and other European countries use its airports as stopovers to get passengers moving again.
