Skies over part of Scotland and northern England have reopened to air traffic, this morning, but the Met Office warns the situation "remains dynamic".
BAA reopened its three Scottish Airports (Glasgow, Edinburgh and Aberdeen) at 7am this morning. Along with Inverness, these airports will remain open until 1am tomorrow morning. Glasgow airport has since closed, but may reopen later today.
The airport operator said it is a "phased opening", with limited flights operating until other destinations are cleared and has advised passengers to check the status of their flight with airlines before travelling to any of its airports.
Airspace restrictions remain in place at BAA's English Airports - Heathrow, Stansted and Southampton, although most European airspace has opened in the last few hours.
Manchester airport said it does not expect to open before 7pm this evening, at the earliest.
British Airways has cancelled all of its short haul flights today, but hopes to operate longhaul flights scheduled to depart after 4pm, if airspace reopens.
The carrier has 12 long-haul flights currently en route to Heathrow. It said contingency plans were in place if the aircraft were to be denied landing permission later in the day.
A statement released by BA in the last hour seemed to bemoan the fact UK airspace remained closed while European carriers were being granted permission to fly.
"Despite the fact that airspace over most European countries is open, UK airspace remains effectively closed.
"With other UK airlines, British Airways is working with the Civil Aviation Authority (CAA) to provide data that would enable the Authority to complete a risk assessment relating to the operation of aircraft in UK airspace.
"We hope that on the basis of the data provided by the industry, the CAA will be in a position to direct National Air Traffic Services (NATS) to reopen UK airspace and allow British Airways to operate flights into and out of the UK.
"We will only operate flights where we have assessed the situation to be safe in accordance with all our normal operating procedures, which are based on many years of experience of operating in areas of volcanic activity around the world."
Ryanair has cancelled flights from the UK, Ireland, Denmark, Finland, Norway, Sweden, Belgium, Holland, France, Germany, Poland, northern Italy and the Baltic States until 1pm on Wednesday (April 21).
The civil aviation authority (CAA) has also decided flights above the cloud, so higher than 20,000ft, will be allowed in all of UK airspace, until 1am tomorrow morning.