Ash cloud footage and advice from Met Office Scientist Derrick Ryall
Airspace in Scotland, Northern Ireland and parts of northern England is due to reopen after five days of travel chaos caused by a volcanic ash cloud.
The air traffic control body, Nats, said from 0700 BST on Tuesday airspace as far south as a line between Teesside and Blackpool would reopen.
Nats said restrictions to airspace above the rest of England and Wales could be lifted later in the day.
Cobra, the UK's emergency committee, has held its second meeting of the day.
A Downing Street spokesman said the latest one, chaired by Gordon Brown, lasted 45 minutes.
"The group welcomed the availability of increased cross-Channel crossings, including at the port of Calais," he said.
"The committee were clear that in light of the ongoing uncertainty about the situation regarding flights, the government should continue to do whatever it can to help get stranded Britons back to the UK using other means.
"This includes making additional ground transport available and providing consular assistance to individuals."
Two Royal Navy warships are being sent to Channel ports to help bring home some of the tens of thousands of British nationals stranded abroad.
Another meeting of Cobra is planned for Tuesday.
Meanwhile, Germany, France and Belgium have said they will begin to reopen airspace from Tuesday.
The EU said it would reduce a no-fly zone in Europe's airspace. Transport minsters said there would be a core no-fly zone, another zone open to all flights and a third "caution" zone, allowing some flights.
In a statement, Nats said mainland Scottish airports would be open.
"The volcanic eruption has reduced and the volcano is not currently emitting ash to altitudes that will affect the UK," he said.
"Assuming there are no further significant ash emissions, we are now looking at a continuously improving situation.
"This is a dynamic and changing situation and is therefore difficult to forecast beyond 0700 local.
"However, the latest Met Office advice is that the contaminated area will continue to move south with the possibility that restrictions to airspace above England and Wales, including the London area, may be lifted later tomorrow."
The next airspace announcement is due at about 2130 BST.
Air travellers, due to fly into reopened airspace, are being advised to check the status of their flight before travelling to the airport.
A spokesman for Glasgow Airport said all Scottish airports were currently working with airlines to understand their plans to operate flights.
Other airports, in more southerly parts of the UK, announced plans to reopen in the hope conditions would continue to improve and restrictions would be lifted.
Manchester Airport said it planned to reopen from 0900 BST on Tuesday unless conditions deteriorated, while East Midlands Airport will reopen at 1200 BST. Leeds Bradford Airport also said it expected some domestic flights to resume on Tuesday.
British Airways said it would aim to resume some flights into and out of London's airports from 1900 BST on Tuesday.
In a statement, it said: "We will aim to operate long haul departures that were scheduled to depart after 1600 BST and short haul departures scheduled to depart after 1900 BST.
"This will however be subject to the full and permanent opening of airspace. All flights before these times have been cancelled."
Ryanair chief executive Michael O'Leary, said he hoped to get flights going again "by lunchtime Wednesday" and clear the backlog "within a day or two".
Airline Flybe said it would start operating services from Aberdeen, Belfast City, Edinburgh, Glasgow, Inverness and Newcastle airports from 1005 BST on Tuesday.
BMI flights are cancelled until 1300 BST, while Jet2 and Thomson flights will continue to be cancelled until Thursday at least.
Planes were first grounded in the UK at midday on Thursday amid fears particles in the ash cloud from the Eyjafjallajoekull volcano could cause engines to shut down.
About 150,000 Britons are currently stranded abroad, according to travel agents' association, Abta.
Earlier, UK emergency committee Cobra discussed options to address the travel chaos.
Speaking later, Prime Minister Gordon Brown said: "I believe this is one of the most serious transport disruptions we have faced.
"It's got financial consequences as well as human consequences and we will do everything in our power to make sure all the arrangements are in place to help people where possible to get back home."
The UK government is deploying three Royal Navy ships to help bring stranded Britons home.
The BBC understands 500 British soldiers from 3rd Battalion, The Rifles are being airlifted by chartered civilian jets from Cyprus to Spain, before sailing to the UK aboard HMS Albion.
Tourists in Calais: 'It took three and a half days to get from Turkey to the Channel'
The ship, due to arrive in the Spanish port of Santander on Tuesday morning, may also carry civilians on its return.
The Ministry of Defence (MoD) said most of the soldiers were coming home from Afghanistan.
Maj Gen Gordon Messenger said there was minimum disruption to ongoing operations in Afghanistan.
But troops were facing extended tours of duty as attempts to bring in replacement soldiers were currently suspended because of the problems caused by the cloud.
He said: "Care of casualties remains an absolute priority and we have an agreement in place with the US to evacuate casualties through their medical chain if necessary.
"We have sufficient stocks and medical facilities in Afghanistan to provide appropriate medical care for anyone who cannot be evacuated. At no stage will the medical care of our injured be compromised."
A spokesman said HMS Ocean was in southern British waters on standby and HMS Ark Royal - deployed on exercise off the north west coast of Scotland - was sailing south to provide further help as required.
The high-speed channel rail link, Eurostar, said it has carried an extra 50,000 passengers in the five days since air travel was disrupted by the ash cloud.
The Met Office has warned the unpredictable nature of the volcano's activity meant there was still cause for concern.
A spokesman said although eruptions subsided on Monday morning to between 4,000 and 5,000ft, they had actually increased during the afternoon to 10,000ft.
At its height, eruptions peaked at 30,000ft, the spokesman said.
On their return from the Easter break, many head teachers in England and Scotland had to cover for absent teachers stranded abroad, as supply agencies struggled to keep up with demand
Air SouthWest carried out
out a 30 minute test flight from Plymouth to Newquay, Cornwall at 1730 BST to assess the impact of the ash cloud
are making continuing to make their own way home, however travellers hoping to return via overland trains from Barcelona through France have found their plans have been scuppered by strikes by employees of the French SNCF rail network
Spain has offered to act as a hub to British travellers trying to get home.
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