British Met Office and aviation authorities have been accused of exaggerating the threat from volcanic ash and sparking an “unnecessary'' crisis by imposing an indefinite ban on flying .

As the crisis dragged on for the fifth day with no hope of an early respite, experts said the Met Office was using a potentially flawed model to assess the risk.

They claimed that satellite pictures did not corroborate the Met's computerised simulation of the ash cloud.

The model used by the Met to forecast the spread of the cloud was said to be based on “probability'' rather than hard evidence and tended to “exaggerate'' the actual size of the cloud , a leading European expert said.

“We don't even know what density the cloud should be in order to affect jet engines.

“We have a model that runs on mathematical projections.

It is probability than actual things happenings,'' said Matthias Ruete, European Commission's Director-General of Transport.

The International Air Transport Association also criticised too much reliance on theoretical modelling and called for a review of the risk-assessment methods.

Its DirectorGgeneral Giovanni Bisignani said: “We have seen volcanic activity in many parts of the world but rarely has it resulted in airspace closures and never on this scale.''

The government, however, insisted that safety was of “paramount concern'' even as airlines and consumer groups threatened to sue for losses and inconvenience because of the crisis.

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