Britain'sCivil Aviation Authority banned all flights from Israel to Britain onThursday after it announced that Europe's busiest airport at Heathrowwill be shut down due to a cloud of volcanic ash covering large partsof Northern Europe.France's Civil Aviation authority also announced that Paris' main airport and nearly two dozen others are also closing due to the volcanic ash. The Civil Aviation authority cites security reasons for the decision.Thousand of flights were canceled, stranding tens of thousands ofpassengers, and officials said it was not clear when it would be safeenough to fly again. In a sobering comment, one scientist in Iceland said the ejection ofvolcanic ash — and therefore possible disruptions in air travel — couldcontinue for days or even weeks.The cloud formed after a huge eruption of a volcano in Iceland earlier this week. Britain's Civil Aviation Authority said non-emergency flights wouldbe banned until at least 6 p.m. (1700 GMT, 1 p.m. EDT). Irishauthorities also closed their air space for eight hours.LondonHeathrow, Europe's business airport, handles upwards of 1,200 flightsand 180,000 passengers per day. The closure also affected London'ssecond- and third-largest airports, Gatwick and Stansted. It was notimmediately clear when flights would resume.With the majortrans-Atlantic hub at Heathrow closed, dozens of flights to the UnitedStates were on hold, and cancellations spread across the continent tomajor hubs at Brussels, Amsterdam, Geneva and Paris, where flightsheading north were canceled until midnight.