Flights to France, Spain, Portugal, Germany and Austria face disruption after a weekend which saw 700 fewer flights than normal, according to Eurocontrol which is responsible for air safety across the continent.
Flights were cancelled from airports across the UK including Belfast, Derry, London Stansted, Bristol, Edinburgh, Liverpool, Manchester and Newcastle. Flights to Geneva, Porto, Barcelona and Nice were among those cancelled.
A spokesman for easyJet said it expected services to France, Spain, Portugal, Germany and Austria to be disrupted again.
A statement on the Gatwick website said the airport was "open for business" but warned flights could be delayed or cancelled.
An ash cloud above north-west Scotland forced the closure of Benbecula and Stornoway airports in the outer Hebrides on Sunday though they later re-opened. Barra airport, also in the outer Hebrides, was also closed for a time.
Several separate ash clouds were lingering in the atmosphere above parts of France, Switzerland, Italy, Germany, Spain and Portugal.
An ash cloud hanging over the Atlantic caused westbound flights to re-route to avoid it.
A spokesman for Heathrow Airport said transatlantic services were suffering delays of up to two hours to allow time to fly around the ash cloud.
Forecaster Rachel Vince, from MeteoGroup, the weather division of the Press Association, said: "The volcano appears to still be erupting but there is a north-easterly wind between the UK and Iceland so the ash is being kept away from the British Isles. As the ash gets to a lower level it is being blown by a westerly wind towards southern Europe."
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