Air travellers hit by the grounding of all flights by the volcanic ash threat can still claim most of their EU rights even though the disruption is beyond airlines' control, the European Commission confirmed.
Travel operators remain liable to reimburse ticket fares or to re-route their stranded passengers - but will not have to pay financial compensation for delays or cancellations, because of the special circumstances.
A range of automatic rights was introduced by EU legislation six years ago, covering meal and accommodation costs if necessary depending on the length of delays, and financial compensation for cancellations or for "denied boarding" - being bumped off overbooked flights.
But the rules say such obligations on air carriers "should be limited or excluded in cases where an event has been caused by extraordinary circumstances which could not have been avoided even if all reasonable measures had been taken".
Those circumstances include "political instability, meteorological conditions incompatible with the operation of the flight concerned, security risks, unexpected flight safety shortcomings and strikes that affect the operation of an operating air carrier".
On Thursday night, with airspace and airports closed in EU countries including the UK, Ireland, The Netherlands, Belgium, Sweden, Denmark and Finland, EU Transport Commissioner Siim Kallas praised the efforts of aviation authorities but made clear that some passenger rights would apply despite the "very exceptional circumstance".
He said the volcanic ash cloud was a very significant threat to air safety and national authorities were required by international law to take steps to ensure safety.
"In this case, the airports and those responsible for air traffic control have taken very swift and appropriate action to safeguard the public, and there is excellent coordination and cooperation at European level, notably within Eurocontrol.
"But, even in exceptional circumstances, EU passenger rights continue to apply and air travellers should speak up to claim their rights."
These include:
• The right to receive information from airlines "on the situation as it evolves, cancellations and length of delays";
• The right to care - "refreshments, meals, accommodation as appropriate";
• The right to choose between reimbursement of fares or be re-routed to final destination.
But Mr Kallas made clear: "In an exceptional circumstance such as this, passengers are not, however, entitled to additional financial compensation that would be the case where delays or cancellations are the fault of the airline."
Earlier, Rochelle Turner,head of research for Which? Holiday magazine, said the delays were more bad news for air passengers in a year that had already seen much disruption to flights.
"Affected passengers are being offered a choice of a full refund or transfer on to another flight. But airlines don't have to compensate passengers for loss of any additional elements to holidays, such as accommodation and car hire.
"It may be possible to claim for these additional elements on your travel insurance, but passengers will need to check their individual policies."
Jennifer Thomas, spokesman for Direct Line Travel Insurance, said: "In the first instance, customers are advised to check with their airline or tour operator.
"If an airline cancels a flight, then customers should claim for a new flight from their airline. If they have booked the flight on their credit card, the Consumer Credit Act could help with any claims made.
"If a customer has a flight cancelled and is refunded from the airline, but still has accommodation that they can't cancel or use, these claims can be considered under travel delay leading to trip abandonment."