Still, authorities say they don't expect the kind of massive grounding that followed last year's eruption of the Eyjafjallajokull volcano in Iceland, because systems and procedures have improved and the cloud is currently not expected to move over continental Europe.
Pilots' unions, however, expressed concerns that the ash could still be dangerous.
Glasgow-based regional airline Loganair canceled 36 flights for Tuesday morning. It said its flights between Scottish islands would be unaffected. Eastern Airways, based in northern England, also canceled flights to and from Scotland on Tuesday.
"Our No. 1 priority is to ensure the safety of people both onboard aircraft and on the ground," said Andrew Haines, Civil Aviation Authority chief. "We can't rule out disruption, but the new arrangements that have been put in place since last year's ash cloud mean the aviation sector is better prepared and will help to reduce any disruption in the event that volcanic ash affects U.K. airspace."
Many airlines said authorities last year overestimated the danger to planes and overreacted by closing airspace for five days amid fears that the abrasive ash could cause engines to stall. CAA spokesman Jonathan Nicholson said authorities this time would give airlines information about the location and density of ash clouds.
The international pilots' federation warned that it believed the cloud still presented a potential danger to commercial aircraft despite developments since last year. "It remains our view that when there is an unknown, then it is always better to err on the side of caution," spokesman Gideon Ewers said.
British forecasters said Monday that there had been no major changes in the forecast; some ash will drift across U.K. airspace, mostly in Scotland and Northern Ireland, by Tuesday morning. But the weather in the United Kingdom has been unsettled in the last two days and will continue that way, making predictions difficult.