European airlines have called for an urgent review into the flight restrictions imposed on 15 April after volcanic ash drifted across the continent from Iceland.
The eruption of Eyjafjallajoekull last week prompted the National Air Traffic Service (NATS) to suspend all air travel within the UK's airspace over safety concerns.
A number of other countries, including Ireland, Germany, Finland and the Netherlands, have closed their airspace, while Italy, France and Poland were among countries to impose a partial shutdown.
However, the European air industry has now called into question the ongoing flight restrictions, with carriers said to be losing more than US$200m (£131.5m, €148.9m) a day in lost revenue.
A joint statement from Airports Council International (ACI) Europe and the Association of European Airlines (AEA) has called for an "immediate reassessment" of the situation, while airlines such as British Airways have successfully staged test flights.
AEA general secretary Ulrich Schulte-Strahaus said: "Verification flights undertaken by several of our airlines have revealed no irregularities at all; this confirms our requirement that other options should be deployed to determine genuine risk."
ACI Europe director general Olivier Jankovec added: "While safety remains a non-negotiable priority, it is not incompatible with our legitimate request to reconsider the present restrictions."
Budget carrier Ryanair has announced it expects cancellations to last until at least 1pm on Wednesday (21 April), while NATS has advised the closure of UK airspace to continue until at least 1am on Tuesday (20 April).
As the restrictions continue to affect leisure and business travel, sporting events have also been hit with the postponement of both the Japanese leg of the MotoGP motorcycling championship due to be held between 23-25 April now postponed.
The final leg of the FINA/Li-Ning Diving World Series - due to be held in Sheffield this week - has also been cancelled, although UEFA has stressed that it expects both Champions League semi-finals to go ahead as planned.
A decision on the Europa League semi-finals - scheduled for Thursday (22 April) - will be made in due course, along with the Futsal Cup finals due to start in Lisbon, Portugal, on Friday (23 April).
