London, England (CNN) -- Major European airports reopened Monday after being closed overnight because of an ash cloud from a volcano in Iceland, but travel remained severely disrupted after the six-hour closure.
Gatwick Airport in London, England, did not accept incoming flights until 11:40 a.m. (5:40 a.m. ET), but the airport expected "knock-on delays and cancellations for the rest of today," the airport's web site said.
Heathrow -- one of the world's busiest airports -- says "delays and cancellations are likely due to restrictions" put in place by air traffic controllers overnight, and urged travelers to check with their airlines before coming to the airport.
In the Netherlands, Amsterdam's Schiphol airport reopened at 1 p.m. local time (7 a.m. ET) after having been closed for seven hours, but "it will take time (until) air traffic will operate according to schedule," it said. And in Ireland, Dublin Airport reopened at noon local time (7 a.m. ET), an airport spokeswoman said.
Britain's Civil Aviation Authority lifted no-fly zones over all of Northern Ireland, airports in Scotland including Edinburgh, Aberdeen and Inverness, and Cardiff in Wales, among others, on Monday at 1 p.m. (7 a.m. ET).
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That left only airports in Scotland's remote Orkney and Shetland islands as the only landing fields closed in the UK, air traffic controllers said.
Eurocontrol, the intergovernmental body overseeing European air traffic, expects about 1,000 flights to be canceled Monday, out of 29,000 on a normal day, it said in a statement.
"During the course of the day, the current cloud is expected to disperse somewhat," the agency said. "By 2 p.m. Central European Time (8 a.m. ET), the cloud is expected to mainly affect Northern Ireland, parts of Scotland and parts of south-west UK.
"There may be some continuing disruption to flights in the greater London area. Delays will also be experienced by flights due to congestion in airspace adjacent to closed areas," the statement said.
The Monday cancellations were the latest in a round of weekend closures of airports across the United Kingdom and into continental Europe.
If the Eyjafjallajokull volcano continues to erupt at current levels and weather conditions prevail, air travel in Britain could be disrupted until Tuesday.
"The government is carefully monitoring this situation and the safety of passengers will remain our paramount concern," said Philip Hammond, transport secretary.
The department said predictions remain fluid and it urged passengers to check with airlines before taking any action. British Airways said it would notify passengers of potential disruptions on its web site.
The problems began in mid-April, when the volcano beneath the Eyjafjallajokull glacier in southern Iceland erupted and sent a cloud of ash into the atmosphere, disrupting international travel for several days.
There's no way to know how long the volcano will continue spewing ash into the air, Georgia Tech scientist Josef Dufek told CNN.
"It could go on another year," he said, noting that an eruption lasted that long in 1820.
Volcanic ash can be a serious hazard to aircraft, reducing visibility, damaging flight controls and ultimately causing jet engines to fail.
British ministers agreed Saturday that five-day ash-prediction charts would now be made available to airlines, other transport providers and the public on the Met Office web site. Previously, only 18-hour forecasts had been available.
