Two of Europe's busiest airports closed early this morning as a dense cloud of volcanic ash drifted from Iceland, aviation authorities said.
The airspace over London's Heathrow Airport closed at 1 a m local time (0530 IST) Britain's National Air Traffic Service said in a statement.
In Amsterdam, Schiphol Airport, another of Europe's biggest air travel hubs, was closed until 2 p m local time today, a statement on Dutch airline KLM's website said.
The restrictions affecting Heathrow -- as well as Gatwick, Stansted and London City airports -- will be in place until at least 7 a m local time (1130 IST) today, the aviation authority said.
Airports across Britain and Ireland were closed for much of yesterday because of the drifting ash. The shifting of the no-fly zone southward will allow airports in northern England -- including the key cities of Manchester and Liverpool -- to reopen after 1 a m local time.
But all airports in Northern Ireland, as well as some Scottish facilities, will remain shut.
In Ireland, Dublin's international airport closed last evening until at least 12 p m today (1630 IST). Some airports in Ireland's west were closed and will reopen at different times today, but Shannon and southern Cork were open "until further notice".
The British air traffic agency said the ash cloud was changing shape and moving south, toward Oxford, England, 60 miles (100 kilometres) northwest of London. Britain's weather service says the northwest winds should shift midweek, redirecting the ash away from Britain.
German authorities sent up two test flights yesterday to measure the ash cloud, one from the German Aerospace Center (DLR) and the other from Lufthansa, the country's biggest airline.
The DLR plane flew to southern England then continued north, collecting data from between 10,000 to 23,000 feet (3,000 to 7,000 meters). The Lufthansa Airbus A340-600, equipped with special scientific gear, left Frankfurt to fly over northern Germany, the United Kingdom and parts of Scandinavia.
All the data from both flights was immediately sent to aviation authorities in the UK, the Netherlands and Germany, said aerospace center spokesman Andreas Schuetz.
Ash can clog jet engines. The April 14 eruption at Iceland's Eyjafjallajokul volcano forced most countries in northern Europe to shut their airspace between April 15-20, grounding more than 100,000 flights and an estimated 10 million travelers worldwide. The shutdown cost airlines more than $2 billion.