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EU transport ministers are to hold emergency talks to ease the paralysis of air travel in Europe caused by the volcanic cloud. It comes as most flights to and from Northern Ireland were cancelled for a fifth day because of the threat posed.
Thousands of passengers remain stranded with planes grounded across Europe.
Air traffic control body Nats has extended restrictions in UK airspace until 1900 BST on Monday. Irish airspace is closed until lunchtime.
More than 6.8m passengers have been affected so far and 63,000 flights have been cancelled since Thursday.
According to the Association of British Travel Agents, about 150,000 people are still stranded.
EU Transport Commissioner Siim Kallas said: "We cannot just wait until this ash cloud dissipates."
Airports and airlines have questioned the need for curbs said to be costing airlines alone $200m (£130m) a day.
Nats said Met Office forecasts showed the ash cloud progressively covering the whole of the UK.
"Anyone hoping to travel should contact their airline before travelling to the airport," it added.
The Irish Aviation Authority said: "Met experts are predicting that the prevailing weather conditions will continue in the coming days. Ongoing restrictions are therefore likely."
Restrictions were lifted in Scotland and Northern Ireland on Friday evening but were reapplied within hours.
Prime Minister Gordon Brown has called a top-level ministerial meeting to discuss the situation.
On Monday, EU transport ministers will be meeting to consider whether flights could resume even if volcanic ash remained in the atmosphere.
'Bizarre'
Belfast International Airport spokesman Uel Hoey said at the weekend that it was "almost surreal" in the airport terminal with "very little activity at all".
"The ironic thing is that since Thursday the airport has remained open to facilitate flights but obviously those flights depend on having safe and conducive conditions for the aircraft to operate so we are very much at the end of the chain," he said.
"The thing in this particular circumstance is while it is extremely bizarre, the frustration is limited with people and they are fairly philosophical because there is an understanding the problem is beyond anyone's real control." Katie Best of Belfast City Airport said staff were on hand to deal with enquiries and help people make alternative arrangements.
"We have staff on the gates at the airport here so people are given an update and told that nothing is flying and that they can make their onward journey from there as appropriate," she said.
Flybe has cancelled all flights due to operate on Sunday and Monday and it has asked passengers not to go to George Best Belfast City Airport.
It said a decision would be taken with regard to Tuesday's schedule by 1000 BST on Monday and passengers are advised to check the airline's website.
Aer Lingus said its UK, European and US flights scheduled to depart on Monday have been cancelled. This includes Aer Lingus regional flights.
Ryanair has cancelled all scheduled flights to and from the UK and Ireland until 1300 BST on Wednesday.
Restrictions on flights in the UK have been in place since 1200 BST on Thursday because of fears particles in the ash from the volcanic eruption in Iceland could shut down plane engines.
Air travel across Europe has been severely affected, with a range of countries from Belgium to Switzerland completely closing their airspace, while others like Austria, Germany and Poland have enforced partial closures.
Experts
Prof Brian Golding, head of forecasting research at the Met Office said it was likely the volcanic cloud would remain over the UK for several days.
"We need a change of wind direction that stays changed for several days and there is no sign of that in the immediate future," he added.
Experts say the tiny particles of rock, glass and sand contained in the ash cloud could jam aircraft engines, as has happened in previous incidents of planes flying into plumes of volcanic ash.
The last eruption of the Eyjafjallajoekull volcano system that is creating the problems was on 20 March, when a 0.5km-long fissure opened up on the eastern side of the glacier at the Fimmvoerduhals Pass.
The eruption prior to that started in 1821 and continued intermittently for more than a year.This article is from the BBC News website. ? British Broadcasting Corporation, The BBC is not responsible for the content of external internet sites.