Flight restrictions affecting Heathrow and Gatwick airports have been lifted, British air traffic control company Nats have said.
In Ireland, Dublin Airport will open at midday, while Donegal airport will be closed until 2pm due to the volcanic ash cloud.
Nats said: “The decision comes following further information from the Met Office about the nature and location of the ash cloud.
“The no-fly zone remains in place in two key areas affecting operations in Northern Ireland and the Shetland Isles. Airports falling within the no-fly zones include Belfast City, Derry, Shetland and Orkney. All other airports are open.”
The Irish Aviation Authority has said the outlook for the coming days is positive and the IAA does not anticipate any further restrictions relating to volcanic ash at Irish airports for at least the next 48 hours.
The IAA said the latest restrictions have been enforced as a result of an increased quantity of volcanic ash over Irish airspace.
The ash plume is also affecting airports in the West with Sligo closed until 10am while Knock Airport has just opened.
Galway and Waterford have reopened, while Cork, Shannon and Kerry remain open.
Britain’s two biggest airports were suffering major flight disruption today - and a string of others were closed – after the return of the Icelandic ash cloud caused chaos for thousands of passengers.
Airports like Manchester, Glasgow and Stansted are open, but passengers everywhere are being urged to check with their airlines before leaving for the airport.
The latest air travel chaos comes a month after the Eyjafjallajokull volcano eruption cancelled a swathe of flights across Europe, and as talks continue to avert a crippling five-day strike by British Airways cabin crew.
There are warnings that the ash restrictions could persist in the UK until tomorrow.
Forecasters are expecting a change in the north-westerly wind direction today, which is expected to blow the “high density” cloud from the Eyjafjallajokull volcano away from Britain on Tuesday.
Earlier revisions to the no-fly zone meant airports in the north of England like Manchester, East Midlands and Leeds Bradford reopened from 1am today, after being closed for most of yesterday.
But Manchester Airport warned: “It is absolutely essential that people contact their airline before travelling to the airport for any flight because there are still high levels of ash contamination above parts of Britain which may bring further disruption.”
Stansted, Luton and East Midlands airports also said they were operational, but all were urging passengers to contact their airlines before setting off.
In Scotland, Prestwick Airport was no longer in the no-fly zone but a spokeswoman said it would not be receiving any flights up until 12.45pm today.
Glasgow airport also said it was open, but warned of knock-on disruption from the other closures, while Aberdeen said it would be shut until at least 1pm.
EasyJet said it was cancelling all flights to and from Edinburgh and Belfast until at least midday.
The carrier said it also expected “an impact” at Gatwick, Bristol, Glasgow, Aberdeen and Inverness airports.
“(We) await further guidance from the relevant airspace authorities,” easyJet said.
British Airways told passengers if their flights were cancelled they could rebook on the next available flight or claim a full refund.
The airline said: “We, like all airlines flying from London airports, are likely to experience significant disruption to our operations (this) morning.
“You are strongly advised to check the status of your flight before heading to the airport.
“We apologise for the inconvenience caused, but the situation is beyond our control.”
The decision to ground aircraft yesterday prompted fierce criticism from Virgin Atlantic president Sir Richard Branson.
He said the closing of Manchester airspace was “beyond a joke”.
“All the test flights by airlines, aircraft and engine manufacturers have shown no evidence that airlines could not continue to fly completely safety,” the entrepreneur said.
“It is obviously dangerous to fly through the mouth of a volcano as has been demonstrated time and time again on television by what happened to the BA plane. However the volcano is hundreds of miles away from the UK.
“Over a thousand flights took off from France last week in similar conditions to that which exist in Manchester today without encountering any problems or showing any levels of ash concentration.
“We need strong leadership to intervene to avoid doing further unnecessary damage to the UK economy and lives of travellers.”
A spokesman for British Airways also said airlines should take the decision whether or not it was safe to fly, describing the current approach as “overly restrictive” and “not justified”.
But CAA chief executive Andrew Haines said: “We are all working flat out to keep flying safe whilst minimising disruption from the volcano. We face a massive challenge to do this.”
He added: “It’s the CAA’s job to ensure the public is kept safe by ensuring safety decisions are based on scientific and engineering evidence; we will not listen to those who effectively say ’let’s suck it and see’.”
Network Rail has pledged to do everything it can to help stranded air passengers make journeys by train.
Virgin Trains said 7,000 extra seats would be provided today, mainly on Anglo Scottish services on two routes – Birmingham to Glasgow and Edinburgh, and London Euston to Glasgow.
There will also be some longer trains on the line between London and Holyhead to connect with ferries to Dublin.