Fourteen thousand flights, approximately half of all Europe's air traffic, were back in the air on Tuesday, as harried air passengers braced for further delays after warnings that a new ash cloud was heading towards Britain.
However, more ash from Iceland's erupting volcano threatened to stall plans to reopen United Kingdom airspace, including at London's Heathrow, one of the world's busiest airports.
However, Britain's Air traffic control company Nats said the 'situation was worsening' in some areas after the eruption of the Icelandic volcano strengthened.
"The volcano eruption in Iceland has strengthened and a new ash cloud is spreading south and east towards the UK," Nats said in a statement.
Meanwhile, Eurocontrol, the intergovernmental body that manages European air travel, said 14,000 flights were expected to operate in European airspace today, which constitute half of scheduled air traffic.
Two-thirds of scheduled flights were cancelled on Monday, and nearly 80 per cent did not fly over the weekend. Flights were restricted to those above 20,000 feet, Eurocontrol said. Planes were back in the air in France, Germany and Italy.
Nearly 75 per cent of scheduled flights were expected from France's two main airports, Charles de Gaulle and Orly, French Minister for Ecology Jean-Louis Borloo told French radio network RTL.
European Union transport ministers, who met on a video link on Monday, reached a deal to resume some flight operations, dividing northern European skies into three zones -- a 'no-fly' zone, a caution zone and an open zone.
The go-ahead for flights came as airlines pointed to successful test flights through closed airspace as a reason for lifting restrictions. Earlier, British Prime Minister Gordon Brown deployed two Royal Navy ships to help bring Britons home, with a third vessel also possibly being deployed. Some European airlines resumed operations, bringing relief to thousands of passengers.
The Nats said test flights in recent days by airlines including KLM, Lufthansa and British Airways suggested that planes can fly safely despite the ash. None of the flights reported any problems or damage.