Flights in Scotland have been cancelled by a regional airline as volcanic ash continues to head towards the UK.
Loganair, based at Glasgow airport, has cancelled 36 flights on Tuesday as the Civil Aviation Authority (CAA) warns that disruption cannot be ruled out.
Analysts expect the ash cloud from an Icelandic volcano to reach Scotland and Northern Ireland by Monday evening.
The event comes a year after ash from the Eyjafjallajokull volcano spread across Europe, causing huge disruption.
Andrew Haines, chief executive of the CAA, said: "Our number one priority is to ensure the safety of people both onboard aircraft and on the ground.
"We can't rule out disruption, but the new arrangements that have been put in place since last year's ash cloud mean the aviation sector is better prepared and will help to reduce any disruption in the event that volcanic ash affects UK airspace."
A Loganair spokesman said Met Office forecasts indicated that a high density of ash would be present in large parts of Scottish airspace throughout Tuesday, clearing into Wednesday morning.
It has cancelled almost all flights on Tuesday and advised customers due to travel to contact them to rearrange flights. Only inter-island routes in Orkney are unaffected.
The UK's air traffic control service, Nats, said volcanic ash was forecast to affect parts of Scotland between 1800 BST and midnight on Monday.
Services may be affected from Barra, Benbecula and Tiree.
The Met Office, which runs Europe's Volcanic Ash Advisory Centre, earlier said there was a possibility of ash moving across the UK towards the end of the week.
But a spokesman said the weather was much more changeable than at the time of last year's eruption and there was a lot more uncertainty.
The CAA said ash levels would be graded as low, medium or high, and airlines would be notified if levels reached medium or high.
Airlines would then consider whether to fly, according to risk assessments already carried out, the CAA added.
NEW technology that could minimise future disruption to planes from volcanic ash was unveiled today by budget airline easyJet. The carrier will be the first airline to trial a new "weather radar for ash" system called AVOID (Airborne Volcanic Object Identifier and Detector).
The system involves placing infrared technology on to an aircraft to supply images to both the pilots and an airline's flight control centre.
These images will enable pilots to see an ash cloud up to 62 miles (100 kilometres) ahead of the aircraft and at altitudes between 5,000ft and 50,000ft.
This will allow pilots to make adjustments to the plane's flight path to avoid any ash cloud.
Millions of passengers had their travel plans wrecked when airlines had to scrap thousands of flights in recent weeks due to Icelandic volcanic ash.
Many UK airports are closed until at least 0100 BST on Monday because of volcanic ash from Iceland, with more disruption likely over coming days.
Manchester, Liverpool, East Midlands, Birmingham, Teesside, Norwich and Doncaster airports are among those closed in England.
Airports in Northern Ireland, Prestwick near Glasgow, some Scottish islands and the Isle of Man are also affected.
London airports are open. Travellers are urged to check with their airline.
The air traffic authority Nats said the Civil Aviation Authority had been forced to extend a no-fly zone imposed earlier on Sunday after the ash cloud had spread further south.
In England, other airports inside the no-fly zone include Humberside, Leeds Bradford, Blackpool, Sheffield, Doncaster and Carlisle.
In Wales, Caernarfon is the only airport affected.
All three Northern Ireland airports - Belfast International, George Best Belfast City Airport and City of Derry Airport - have also been shut.
Spain has closed 15 airports as a cloud of ash from an Icelandic volcano drifts south over Western Europe.
National airport management agency Aena said nine airports closed early on Saturday and six more shut from 1200 local time (1000 GMT).
The restrictions would be in place until at least 1800, Aena said
Most flights between Europe and North America are being diverted because of the ash cloud's latest drifting, officials at Eurocontrol said.
Flights are being rerouted north and south of the 1,200 mile (2,000km) long cloud.
On average, 600 airliners make the Atlantic crossing every day, correspondents say.
Aena said the airports affected were Bilbao, San Sebastian, Vitoria, Zaragoza, Pamplona, La Rioja, Santiago, La Coruna, Vigo, Asturias, Santander, Leon, Valladolid, Burgos and Salamanca.
Eurocontrol, the agency that co-ordinates aviation safety in Europe, said airports were also expected to close in northern Portugal and parts of southern France.
In the UK, some flights to Spain were being affected.
At London Stansted, 22 Ryanair flights to the Canary Islands, mainland Spain and Portugal were cancelled, along with three EasyJet flights.
Flights from Gatwick to Portugal, Alicante and Madrid were cancelled and at Heathrow some flights to La Coruna in northern Spain were also grounded.
Last month, thousands of travellers were stranded after ash shut down airspace across Europe.
Recent images have shown activity in the Eyjafjallajokull volcano intensifying.
Experts at the UK's Met Office said it was sending ash up to heights of 30,000 ft (9,100m).
Flights across Ireland and parts of the UK were disrupted earlier this week.
The ash is back. Fresh volcanic activity under the Eyjafjallajokull glacier in Iceland has resulted in a fresh ash plume being pushed about 5.5km (18,000ft) into the air.