UK and Nato air crews are taking part in Exercise Joint Warrior
All routine military air training in UK airspace has been suspended as a plume of volcanic ash drifts across much of northern Europe.
The Ministry of Defence (MoD) said a decision on the resumption of training would be taken on Friday.
A spokeswoman said the suspension took effect from midday.
Disruption to flights caused by a volcanic eruption in Iceland comes as UK and Nato air crews take part in Exercise Joint Warrior.
Europe's largest military exercise involves 50 aircraft, including fixed wing maritime patrol aircraft flying from RAF stations at Kinloss and Lossiemouth in Moray.
Live firing is also conducted at Cape Wrath on the far north coast of Scotland.
Joint Warrior also features 16 Air Assault Brigade and the Royal Navy's Carrier Strike Group headed by HMS Ark Royal.