Aviation authorities have now extended restrictions on UK airspace

According to the UK's Met Office, the eruption of the Eyjafjallajoekull volcano in Iceland is "weakening", but weather patterns, for the time being, continue to blow volcanic ash towards the UK.
"We will continue to offer advice to Nats (the UK's air traffic control authority) about the spread of any residual ash," the Met Office said on its website.
"Assuming the volcanic activity continues to weaken, we can expect ash clearance across the UK and we will be looking for the earliest opportunity that this might happen."
And according to current European regulations, no matter how low the concentration, aviation authorities will not reinstate normal control over airspace as long as the ash cloud remains.
A spokesperson from from Nats told BBC News that there was "no threshold" for concentrations at which volcanic ash was acceptable.
The dust is simply too dangerous for jet engines to risk commercial flights encountering it, said Nats. If volcanic ash particles are ingested into a jet engine, they accumulate and clog the engine with molten glass, which can cause the engine to shut down.



Because of this, the International Civil Aviation Organization (ICAO) has recommended implementing a no-fly zone if volcanic ash is detectable in airspace. (ICAO operates the International Airways Volcano Watch system).
Whether to open or close airspace is a decision for national aviation authorities, but all European nations abide by these safety recommendations.
Aviation consultant, Chris Yates explained: "The ICAO regulation that has prompted this widespread grounding is from experience gained from over 80 incidents between 1980 and 2000 [as well as] computer modelling and best 'guestimate'."
But the current paralysis of UK and European air traffic is unprecedented, so the regulations may have to be changed in order to get things moving.
"The airline industry...are clearly making the argument that we are being over cautious in grounding all flights," said Mr Yates.
"Whilst it remains possible to find clear air up above us this doesn't necessarily mean that there are no pockets of high concentrations of ash at the various flight levels. I would suggest it's better to err on the side of caution."
But the European Commission has now intervened to organise a meeting on Friday between technical and safety experts to find out if there are other options, such as setting up a fly zone that would allow carriers to navigate around the plume.
But policy-makers have stressed that any decision must be based on a scientific assessment of where and when it is safe to fly.

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