Air travel restrictions across Northern Europe are now into their fifth consecutive day.
Some 150,000 British business and leisure travellers are believed to be still stranded abroad.
Air traffic managers Nats said a UK airspace will be locked down until at least 7pm tonight (April 19).
UK airspace has been closed since Thursday after a Volcano in Iceland erupted and volcanic ash spread across the UK and Europe.
Last night British Airways completed a two-and-three-quarter hour test flight from London to Cardiff.
KLM has also completed test flights in Europe.
Airlines are estimated to be losing some £130 million per day while the travel ban remains in place.
They are questioning whether or not the ban should remain in place.
Train and ferry companies have reported unprecedented levels of traffic.
Eurostar is running up to eight extra services.
More than 6.8 million passengers have been affected across Europe.
Travel management companies worked over the weekend to try an repatriate clients.
Airspace closed:
Austria, Belgium, Czech Republic, Denmark, Estonia, Finland, Germany, Hungary, Ireland, Latvia, Luxembourg, Netherlands, Poland, Romania, Slovakia, Switzerland, UK
Partial closures:
Bulgaria (Sofia and Plovdiv open)
France (southern airports open)
Italy (northern airspace expected to reopen from 0500 GMT Monday)
Norway (most airports open)
Poland (several airports, including Warsaw, open)
Sweden (northern airports open)
Flights operating:
Greece, Lithuania, Portugal, Russia, Turkey, Ukraine, Spain