Adds Air Malta statement:
Hundreds of flights to the UK have been cancelled after fine volcanic ash, invisible in most cases, drifted into British airspace this morning.
Flights in Norway and Northern Sweden were also affected.
EasyJet and Ryanair cancelled flights to Malta and Air Malta said its flights from the UK to Malta may be affected.
108 flights were cancelled at London's Gatwick airport and 150 from Heathrow as a result of the unusual event, although the airports remained open.
Hundreds more flights were affected in Stansted, Manchester, Newcastle, Belfast, Edinburgh, Aberdeen, and Glasgow. Some of the airports were closed. The disruption is expected to continue into the afternoon, particularly in Scotland.
EasyJet flights to Malta from Manchester and Gatwick were cancelled. Ryanair cancelled a flight to Malta from Dublin. An Air Malta flight to Gatwick departed at 8.49 a.m. All London airports are expected to close at 12.30 p.m.
Air Malta said its flights to Heathrow (KM100) and Gatwick (KM116) operated normally out of Malta.
Air Malta’s flights KM146/7 to/from Manchester, KM172/3 - to/from Birmingham, KM102/3 to from London Heathrow and KM118/9 to/from London Gatwick later on today maybe affected.
"Air Malta is closely monitoring the situation and will advise its passengers of any changes in its flight operations as a result of these airspace restrictions," the airline said.
Due to this situation, clients were therefore advised to contact flight enquiries of the Malta International Airport on tel: 5004 3333 for further information and to confirm flight arrival / departure times.
A spokeswoman for EasyJet speaking in London said: "Following the eruption of a volcano in Iceland yesterday, an ash plume has entered UK and Scandinavian airspace overnight.
"As a result NATS have closed parts of UK airspace and this is causing significant disruption to all airlines due to operate flights to and from the UK today.
"EasyJet passengers are advised to check the website before they leave for the airport for any disruption information."
Forecasters believe the ash could take a number of days to disperse. Fine volcanic ash is a hazard for aircraft because it is sucked into engines and blocks air flow.
Matt Dobson, a forecaster for MeteoGroup, the weather division of the Press Association, said: "The concern is that as well as the eruption, the jet stream passing through Iceland is passing in a south easterly direction, which will bring ash to the north of Scotland and Denmark and Norway. But it is impossible to say how much ash will come down.
"It could be a threat in these areas from now until tomorrow or Friday."
A spokesman from Nats said: "The Volcanic Ash Advisory Centre has issued a forecast that the ash cloud from the volcanic eruption in Iceland will track over Europe tonight.
"NATS is working with Eurocontrol and our colleagues in Europe's other air navigation service providers to take the appropriate action to ensure safety in accordance with international aviation policy."