Ryanair has denied ash was found in any of its planes
Ryanair has said that volcanic ash has been found in the engines of two of its aircraft at Belfast City Airport.
Four Ryanair flights to England were cancelled at the airport on Sunday, leaving dozens of passengers stranded.
Initially, the airline said the planes all had separate technical problems unrelated to the Icelandic eruption.
But after further tests on Sunday, the airline confirmed that two of its aircraft at the City airport showed small traces of ash in their engines.
A Ryanair spokesman said the planes would return to service later on Monday.
"These aircraft will return to service once the manufacturer's approved procedures for return to service in such cases has been completed by Ryanair's engineers," he said.
He added that there "was no risk and no cause for concern."
The planes had flown in UK airspace which was open and unrestricted at the time.
The spokesman said Ryanair cannot explain why there was ash in the engines other than there are trace elements of it in the atmosphere.
UK airspace was closed for six days in April because of the ash cloud from Iceland's volcano.
The ash cloud has continued to cause disruption to flights in Europe.
Airports in Portugal, France and Austria have all been forced to ground some flights overnight, while some airports in Scotland and the Republic of Ireland had restrictions placed on them over the weekend.
Travellers from Northern Ireland are continuing to experience difficulties as a result.
Orla McCabe from Dunmurry is stranded at Faro airport in Portugal.
She is pregnant and her party of eight contains three children and a disabled person.
She said their Easyjet flight was cancelled on Monday morning and all other flights back to Belfast with that airline are full until Saturday.
She said she has been told they will need to pay for additional food and transfers and claim the money back later.
The Met Office has also warned that the ash may return to UK airspace in the coming days.