Thousands of passengers were forced to abandon air travel plans after ash from an Icelandic volcano caused the cancellation of 500 flights in Europe on Tuesday.
Ash from the plume which has been rising from the Grimsvotn volcano since Saturday has drifted south into Scotland and parts of northern England.
Germany was closing the airspace over northern cities, including Bremen and Hamburg, on Wednesday.
European air traffic agency Eurocontrol said about 500 flights had been cancelled on Tuesday.
Airports in more than a dozen UK towns and cities, including Londonderry, Glasgow and Edinburgh, have either cancelled or delayed their flights.
There were no immediate reports of threats to largest airports such as London Heathrow, but controllers said Denmark, Norway and Sweden could see some disruption by Wednesday.
Eurocontrol, the Brussels-based agency responsible for some of the world's busiest air corridors, said the impact on flights was expected to be far lower than during the Icelandic volcano eruption last year, due to new procedures.
However the head of the Irish budget airline Ryanair said the authorities' approach was "bureaucratic bungling".
Chief executive Michael O'Leary says his company flew a test flight over Scotland and saw no evidence of volcanic ash.
"Even today, the Icelandic airports are open but they've closed the Scottish airports.
"The airlines are the best people to prioritise safety, not some idiot in the UK Met Office," he said.
The International Air Transport Association (IATA) said Europe is handling the volcanic ash crisis better than last year but governments need to agree on how to regulate their airspace to avoid a patchwork response.
In April last year, tens of thousands of flights were grounded after the eruption of the Eyjafjallajokull volcano, also in Iceland.
