An Icelandic volcano is erupting again, sending a new ash cloud towards Britain, the country's air authorities say.
"The volcano eruption in Iceland has strengthened and a new ash cloud is spreading south and east towards the UK," said the National Air Traffic Services (NATS), which manages British airspace.
The change means it is unclear whether UK airspace will be reopened later as planned, in an attempt to ease global travel chaos caused by the ash cloud.
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"Latest information from the Met Office (weather forecasting service) shows that the situation is worsening in some areas," said NATS.
It added that Scottish airports should still be open from 7am (0600 GMT) Tuesday, as had been announced earlier, but that the situation for Northern Irish airports was uncertain.
More airspace over England may become available from 1pm (1200 GMT) although not as far south as the main London airports, said NATS.
But it added that "the situation is likely to change overnight".
The air traffic control service said earlier that Britain would begin lifting a flight ban Tuesday that had been in force since last week, leaving tens of thousands of travellers stranded abroad.
British Airways had said it hoped to resume flights into and out of London, in the southeast of England, from 7pm Tuesday (0400 AEST on Wednesday). But after the latest NATS update, BA said it was "reviewing" its schedule.
The latest development showed "the dynamic and rapidly changing conditions in which we are working," said the air authority, announcing a fresh statement for 3am on Tuesday (1200 AEST).
A meeting of the British government's emergency planning committee, Cobra, chaired by Prime Minister Gordon Brown was held late on Monday local time to discuss the latest updates on the ash cloud.
Brown announced earlier that navy ships were being sent to help bring Britons stranded abroad back home.
