Air traffic control company Nats said the "situation was worsening" in some areas.

But it said Scottish airports should be available from 7am on Tuesday and other airspace over England from 1pm, although that does not include the main London airports.

Nats said in a statement: "The volcano eruption in Iceland has strengthened and a new ash cloud is spreading south and east towards the UK.

"This demonstrates the dynamic and rapidly-changing conditions in which we are working.

"Latest information from the Met Office shows that the situation is worsening in some areas.

"Based on this information, the situation for Northern Irish airports for the morning is uncertain, due to the new ash cloud.

"The latest information shows that Scottish airports should be available from 7am and more airspace over England may become available from 1pm, although not as far south as the main London airports."

Earlier, Prime Minister Gordon Brown said that two Royal Navy ships were to be used to help bring Britons home, with a third vessel also possibly being deployed.

The go-ahead for flights came as airlines pointed to successful test flights through closed airspace as a reason for lifting restrictions.