Several British and Irish airports closed Sunday as the ash cloud returned, drifting south following fresh eruptions from the Icelandic volcano.
Manchester Airport in northwest England and Dublin Airport, among the 20 busiest in Europe, were among those affected by the cloud, with the ash levels deemed too dangerous to fly through.
Manchester - Britain's busiest airport outside London, where airports were so far unaffected - was among a host of British air hubs to shut until 1:00 am (0000 GMT) Monday.
Dublin, Ireland's main airport, was to close from 1800 GMT until at least 0800 GMT Monday.
Europe's skies were partially closed for up to a week last month following the eruption of Iceland's Eyjafjoell volcano, in the biggest shutdown of the continent's airspace since World War II.
The volcanic ash can cause serious damage to jet engines.
A University of Iceland vulcanologist said Eyjafjoell activity had recently risen.
"There is slightly increased activity for the past two days, there has been some ash fall around the glacier," said Bjoern Oddsson.
"The column (of smoke) has increased and rises up to eight kilometres (five miles)," he told AFP, as opposed to six kilometres in previous days.
Its effect on European flights "all depends on the winds", said the geologist.
In England, several other airports were shut until 0000 GMT Monday as the no-fly zone extended southwards.
They included regional air hubs Birmingham, East Midlands, Liverpool, Leeds-Bradford, Doncaster, Durham Tees Valley, Norwich and Humberside.
In Scotland, the main Edinburgh and Glasgow airports were open but the fourth busiest, Prestwick, and some island airports shut down, while all Northern Ireland airports closed for the period. The Isle of Man airport was also shut.
"Predictions suggest the situation is likely to worsen over the next 24 hours before easing into Tuesday," said Scottish Finance Secretary John Swinney.
National Air Traffic Services, which manages British airspace, said: "The ash cloud continues to change shape and move further south".
NATS added that at least until 0000 GMT Monday, "London's main airports will still be clear of the no-fly zone imposed by the Civil Aviation Autority due to the high density volcanic ash cloud."
Britain's Department of Transport has warned that British airspace is likely to face partial closures from Sunday until Tuesday.
London airports, including Europe's busiest air hub, Heathrow, could be affected, it said.
Airlines bridled at the restrictions.
A British Airways spokesman said airlines should decide whether it was safe to fly, calling the current approach "overly restrictive" and "not justified".
Virgin Atlantic boss Richard Branson branded the closure of Manchester "beyond a joke".
"We need strong leadership to intervene to avoid doing further unnecessary damage to the UK economy and lives of travellers," he said.
Eurostar, which runs high-speed rail services linking London with Paris and Brussels via the Channel Tunnel, said it was laying on extra trains between the British and French capitals Monday to answer an expected surge in demand.
In the Republic of Ireland, of the main three airports, Dublin was to close from 1800 GMT until at least 0800 GMT on Monday, while Cork and Shannon remain open until further notice, the Irish Aviation Authority said.
Of the smaller hubs, Ireland West (Knock), Donegal and Sligo airports remain closed until at least 1100 GMT Monday.
Kerry is open until further notice, Galway is closed until at least 0800 GMT Monday and Waterford is to close from 2200 GMT until at least 0800 GMT Monday.
North Atlantic flights crossing Irish-controlled airspace remain unaffected.
In Scandinavia, the skies were open over Denmark, Finland, Norway and Sweden and should remain so until at least Monday night, when there is a chance of Danish airspace being affected.
There were no restrictions on German flights, with the DFS air safety authority saying that ash pollution was "weak" for the moment. Weather forecasts indicate it should remain that way until at least Tuesday.Reuse content