LONDON >> Airlines clashed with regulators and pilots Tuesday as passengers braced themselves for days of uncertainty and chaos caused by a volcanic ash cloud being blown across the British Isles.

Britain’s weather service said the ash from Iceland’s Grimsvotn volcano had moved over parts of Scotland, prompting air regulators to warn airlines that they had to seek permission to fly to and from the area. Hundreds of flights were canceled in Britain and Ireland.

Spokesman Barry Grommett said volcanic ash had been detected on the ground in Scotland, and a specialist laser based on the Shetland Islands northeast of the Scottish mainland had also detected ash in the atmosphere.

"All the data we are receiving confirms our forecasts, that there is high-density ash over Scotland," Grommett said. Experts say that particles in the ash could stall jet engines and sandblast planes’ windows.

Irish budget airline Ryanair immediately challenged the results. It said it had sent its own airplane into Scottish airspace and found no ash in the atmosphere.

"Exactly as we predicted, we encountered absolutely no problems, Ryanair’s chief executive Michael O’Leary told The Associated Press. "There’s no cloud over Scotland. There’s no dusting of ash on the airframe or the wings. The airspace over Scotland should never have been restricted in the first place."

The main international body representing carriers, the International Air Transport Association, complained to the British government about the way it had handled the issue, saying it should have had Cessna planes ready to carry out tests, instead of relying on the weather service.

But other airlines were more willing to follow official advice. Declan Kearney, spokesman for Aer Lingus, said it had canceled 20 flights between Ireland and Scotland.

"We take the advice given to us," he said. "We have no reason to question the advice being given to us by the aviation authorities at this time. We need to accept what the experts in this area are telling us."

The European Cockpit Association that represents pilots warned airlines against allowing their planes to venture into areas with even moderate ash concentrations.

The association "cannot accept under any circumstances any flights into the red zone, even if these are approved by airlines," said secretary general Philip von Schoppenthau.

Britain’s Civil Aviation Authority said the high-density ash was in the skies above parts of Scotland by late Tuesday morning, and that it was likely to affect northern England and Northern Ireland in the afternoon.

The Grimsvotn volcano began erupting on Saturday, sending clouds of ash high into the air that have then been carried toward the British Isles on the wind.

Brian Flynn, head of network operations at Eurocontrol said between 200 and 250 flights had already been canceled on Tuesday, and warned that up 500 flights could be affected.

British Transport Secretary Philip Hammond said there had also been some "modest delays" to flights crossing the Atlantic, as aircraft need to avoid areas of high ash concentration. He added that there may be more delays later this week.

The ash cloud forced President Barack Obama to shorten a visit to Ireland on Monday, and has raised fears of a repeat of huge travel disruptions in Europe last year when emissions from another of Iceland’s volcanos, Eyjafjallajokull, stranded millions of passengers.

Last year, European aviation authorities closed vast swaths of European airspace as soon as they detected the presence of even a small amount of volcanic ash in the atmosphere. This year, they are trying a more sophisticated approach.

Aviation authorities will give airlines information detailed information about the location and density of ash clouds. Any airline that wants to fly through the ash cloud can do so, if they can convince their own national aviation regulators it is safe to do so.

Other airlines decided not to take any risks. British Airways suspended all its flights for Tuesday morning between London and Scotland, while Dutch carrier KLM and budget airline easyJet canceled flights to and from Scotland and northern England at the same time. Three domestic airlines also announced flight disruptions.

The closures are already affecting travel plans, In Ireland, a couple who were due to fly to Edinburgh for a friend’s wedding were told their flight had been canceled. Anne and Damien Farrell decided on the spot to reclaim the car they’d just parked in Dublin Airport’s long-term parking lot, drive the 100 miles (160 kilometers) north to Belfast, and take the ferry to the Scottish port of Stranraer.

"Fortunately we have a day of lead-in time before the wedding party gets going, otherwise we’d be up a certain creek without a paddle," said Damien Farrell, 29.

Norwegian airport operator Avinor said the ash cloud that swept over southwestern Norway earlier Tuesday has now moved away from the coast and no longer affects the airports in Stavanger and Karmoey. However, it said the ash is expected to return to southern Norway in the afternoon.

In Brussels, European Union transportation commissioner Siim Kallas said passengers and airlines were facing "a very challenging week."

"Although we are partly dependent on the weather and the pattern of ash dispersion, we do not at this stage anticipate the widespread airspace closures and the prolonged disruption we saw last year," Kallas said.

Eurocontrol said the ash was not expected to affect trans-Atlantic flights because it was mostly north of the usual routes and was not reaching above 35,000 feet. Airliners crossing the Atlantic usually cruise at altitudes above 37,000 feet, where the thin air density reduces fuel burn.

Spanish airport authority Aena said two flights to the UK were canceled Tuesday morning. Spanish soccer team Barcelona plans to travel to London on Thursday for Saturday’s Champions League final against Manchester United at Wembley Stadium, though club officials say they are monitoring the ash cloud disruption and could change their departure date.

Last year, Barcelona traveled to Italy by bus for a Champions League semifinal at Inter Milan because of the closure of European airspace.

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Lekic reported from Brussels. Shawn Pogatchnik in Dublin contributed to this report.