Many of Europe's busiest airports are back up and running, following a weekend of delays due to volcanic ash.

The dense cloud lingering from an Iceland volcano caused no fly zones to be set up around northern Europe through early Monday.

However, Britain's Civil Aviation Authority eventually agreed with airlines to relax flight safety rules that allowed planes in and out of the area.

"There is mounting evidence that aircraft can fly safely through areas of medium density, provided some additional precautions are taken. This is now what has been agreed," explained Richard Deakin, CEO of the British air traffic control company NATS. "As a result of this change, there are no predicted restrictions on U.K. airspace in the immediate future."

Before the change, planes were grounded at busy airports throughout the region, including London's Heathrow Airport.

There were about 1,000 less flights than expected in Europe Monday. Volcanic ash caused more than 100,000 flights to be grounded in April.