Air travel chaos: Volcano has 'effectively stopped' erupting

The Volcanic Ash Advisory Centre, based at the Met Office, said only small amounts of ash were now being ejected up to 6,000 feet in the atmosphere.

The Met Office said this was "cause for well-grounded optimism" but warned that time was needed to ensure the ash cloud was dispersed.

"Cutting off the source is terrific but what it then needs to do is disperse, it just needs time," a spokesman said.

"So it is not an immediate switch on, switch off."

Dr David Rothery, of the Open University department of earth and environmental sciences, also said there were "grounds for cautious optimism" after observing a "change in character" in the volcano's activity from webcam pictures.

He said: "There was no high ash column rising above the vent.

"If this situation persists, then the high altitude ash cloud will be starved of fresh ash, and will eventually disperse.

"What has probably happened inside the volcano, is that meltwater from the ice-cap is no longer able to leak into the magma conduit, where it has been the main driving force for the explosive expansion that has hitherto, since Thursday, been responsible for the eruption column."

He added that there was "no guarantee" that the situation would not revert to what had been happening since last Thursday, but he said there were grounds for "cautious optimism".

David Murphy, head of Met Eireann aviation services division in Ireland, said he was optimistic the situation would dramatically improve.

"If the eruption has ceased and remains stopped then the current ash plume will probably be blown away from the UK and Ireland overnight, by tomorrow night at the latest.

"There's a good chance that Ireland and UK airspace, or most of it, will be clear by some time coming into tomorrow evening or at the latest the following morning."

Meanwhile, spectacular pictures have been captured of lightning displays over the Eyjafjallajkull volcano.

Lightning is often common above volcanoes. Clouds of toxic ash and dust ionise the air, generating an explosive electrical storm.