Flight restrictions have been lifted at Heathrow, Gatwick and other airports after closures prompted by the return of a volcanic ash cloud to UK airspace.
After many airports faced restrictions over the weekend and into Monday, just two are currently in the no-fly zone: Shetland and Orkney.
The industry has warned that knock-on disruption could last for some hours.
British Airways boss Willie Walsh called the latest restrictions "a gross overreaction to a very minor risk".
Airport operator BAA, which runs six UK airports including Heathrow and Stansted, said: "It is likely that some delays and cancellations will go on throughout the day and we continue to advise passengers to check the status of their flights with airlines."
In the Netherlands, Amsterdam's Schiphol and Rotterdam airports reopened from 1300 local time (1200 BST) after being closed for seven hours.
Flights in and out of Dublin, in the Irish Republic, resumed from noon.
The latest dense patch of ash disrupted the travel plans of tens of thousands of people over the weekend, mainly in northern parts of the UK.
Network Rail pledged to do everything possible to help stranded and delayed travellers make journeys by train.
Virgin Trains said 7,000 extra seats would be made available on Monday, mainly on routes between Birmingham and Glasgow and Edinburgh, and between London Euston and Glasgow.
Eurostar says it is laying on six extra trains through the Channel Tunnel on Monday, amounting to about 5,500 additional seats.
Ash from the Eyjafjallajokull volcano has led to thousands of flights being delayed or cancelled across Europe since April.
The latest UK disruption saw airspace over Northern Ireland close first on Saturday, before the cloud moved south and grounded flights in many parts of the UK on Sunday.
BA boss Willie Walsh says th ash risk can be managed with closing airspace.
Among the affected travellers who contacted the BBC News website was Matt Pope, from Guildford, who e-mailed saying: "This is now the third time the ash has disrupted my travel plans. The first occasion I was stuck in North Carolina for six days.
"Last weekend the Easyjet flight from Prague to Gatwick was cancelled due to aircraft positioning problems after ash in central Europe. This was after we ran the marathon and I missed my flight to Singapore the next day causing expensive rescheduling.
"Now I am sat at Heathrow awaiting for a flight to NY. Will this ever end?"
Safety fears are due to the possibility ash could turn into molten glass as it hits the heat of plane engines, crippling aircraft.
Restrictions depend on the density of ash in airspace, and the threshold level that forces a ban was raised by the Civil Aviation Authority (CAA) following six days of airport closures last month.
But after the latest airport closures this weekend, airlines have criticised the amended regulations.
BA chief executive Mr Walsh said: "I am very concerned that we have decisions on opening and closing of airports based on a theoretical model.
"There was no evidence of ash in the skies over London today yet Heathrow was closed."
He added that airlines flew safely in other parts of the world where there was volcanic activity, saying: "If we can do it in every other part of the world, I can assure you we can do it in the UK as well."
On Sunday, Virgin Atlantic president Sir Richard Branson called the closure of Manchester airport "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 safely," he said.
Transport Secretary Philip Hammond has told the BBC the government and airline industry are working together on how to enable more flights to operate while there is ash in the atmosphere.
"The threshold at which air is considered unsafe to fly through has already increased 10-fold from a 200-microgram limit to a 2,000-microgram limit, and that was agreed with the airlines after the last period of closure," he told BBC Radio 4's Today programme.
"Work is ongoing with the airlines, with the aircraft manufacturers, with the engine manufacturers, to see if a safe operating regime could be introduced at a yet higher threshold of ash, with enhanced, more regular engine inspections.
"We are not there yet but we are optimistic that progress will be made and that, of course, would have a very significant impact on the level of disruption that the continuing volcanic eruption is having on UK airspace."
CAA chief executive Andrew Haines said: "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.'"