The ash cloud from the erupting Grimsvotn volcano in Iceland is expected to disrupt U.K. flights from Tuesday, the Civil Aviation Authority said.
The latest Met Office predictions showed the plume of ash would cover the Irish Republic, Northern Ireland, Scotland and parts of northern Britain early on Tuesday morning.
At its densest the cloud is likely to exceed the top threshold set by the aviation industry for aircraft safety.
Asked whether this would cause some disruption to flights, a CAA spokesman said: "That's the way it's looking certainly at the moment."
U.S. President Barack Obama is due to fly to Britain on Tuesday morning from Ireland for a state visit.
Last year an eruption by another Icelandic volcano caused six days of travel chaos but the CAA said lessons learned from that incident would reduce the impact this time.
"We know now that engines can cope with certain concentrations of ash. That's going to be the key difference," said the CAA spokesman.
He added airlines that presented a safety case would be allowed to fly aircraft through "medium density" ash clouds.
"On that basis we hope that some flights will be able to take place in that kind of medium zone around the densest concentrations of ash. We aren't certain yet what we're up against," he said.
A spokesman for Britain's Met Office said they expected winds to continue from northwest until about lunchtime on Tuesday, but weather conditions, including rain, were predicted to help.
"All sorts of things point to a relatively (optimistic outlook) I suppose in terms of what's happening with the weather patterns and the pressure unlike last year," he said.
The Grimsvotn volcano began erupting on the weekend and forced Iceland to close its airspace on Monday.
The country's aviation authority said it hoped to reopen the main airport near the capital Reykjavik later today or tonight.
The only other country to be affected so far is Greenland, with air traffic officials confirming airspace was partly closed over the Arctic island.
The damage caused by the latest eruption is not expected to wreak the same havoc as the Eyjafjallajökull eruption in April 2010.
Not only is the ash of a different consistency and less likely to cause damage to aircraft, but aviation rules have been changed to allow airlines to continue to fly if they believe it is safe to do so.
Andrew Haines, chief executive of the CAA, said: "Our number one priority is to ensure the safety of people both on board 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."
A new radar system in Iceland means the Met Office has a "far better model", which means planes can be directed more expertly around UK airspace.
The Government has also insisted this time round, it can deal with the situation.
Transport Secretary Philip Hammond said: "We've spent a lot of time and quite a bit of money putting in place the equipment that's necessary.
"[We've been] working with the airlines, the Civil Aviation Authority, the air traffic control people, so we do actually now have a system worked out."