LONDON—Airlines that are losing revenue from flight cancellations caused by a cloud of volcanic ash over much of Europe are unlikely to have insurance to recover those losses, sources say.

Flights remained grounded Friday at many European airports as the ash from the eruption of Iceland’s Eyjafjallajökull volcano drifted across the region, endangering aircraft.

Airlines probably are not covered for revenue lost due to grounded aircraft in this case, said Steven Doyle, London-based business development director of aerospace at Willis Aerospace, a division of broker Willis Group Holdings P.L.C. “That would be my assumption because there is no physical damage,” he said.

Business interruption policies generally are triggered only when physical damage leads to a loss of revenue.

The ash—consisting of fine particles of glass and rock—could damage aircraft engines and other parts, possibly resulting in engine failure, said Paul Hayes, director of air safety at Ascend Worldwide Ltd., a London-based aerospace consultant.

“It can get everywhere in an engine, melt and plate onto different sections,” Mr. Hayes said. “It would also sandblast all leading edges; the windscreen could become opaque.”

Mr. Hayes said there have been several instances since 1982 when aircraft have flown through ash clouds and experienced engine failure. “Fortunately all the aircraft managed to land safely,” he said.

The disruption in aviation caused many travelers who purchased travel insurance to turn to their insurers to determine whether the cost of alternative transportation and other extra expenses are covered.

The Assn. of British Insurers in London warned that cancellations and delays caused by volcanic eruptions might not be covered by travel policies. If a flight is canceled and a policyholder took out single-trip insurance, travel policies generally respond, the ABI said in a statement.

“Travel insurance policies will differ in this situation; there is no standard set of conditions which applies to a situation of this kind,” Nick Starling, the ABI’s director of general insurance and health, said in the statement.