The Met Office has defended its forecasting after claims that the ash cloud that caused flight chaos on Monday "did not exist".

British Airways chief executive Willie Walsh was among airline leaders highly critical of the Met Office forecasting which led the Civil Aviation Authority to impose some no-fly zones on Monday.

Mr Walsh said there had been no evidence of ash in the skies over London yet flight restrictions had been put in place at Heathrow for the first part of the morning.

But Met Office senior press officer John Hammond said: "There was ash over the UK. Our forecasts are updated regularly and are based on a number of factors, including observations from space and inputs from our colleagues in Iceland. I cannot stress enough just how changeable the ash-cloud situation is. There is change hour by hour and we try to feed through as much information as we possibly can."

Mr Hammond added: "The good news is that the remains of the ash cloud are moving away from the British Isles and we will be getting winds that are coming up from the south west. The problem has been that we have been having long periods where winds have come from the north west and this has blown the ash over the British Isles."

The threat of flight restrictions from volcanic ash has been reduced with the introduction of new measures allowing planes to fly when there is more ash in the air.

After weeks of misery for airlines and passengers, the new Time Limited Zone (TLZ) will allow airlines to resume a more normal schedule because they can now operate in all but the densest ash clouds.

The new rules have effectively doubled the safe operating threshold of ash acceptable by engines, the Civil Aviation Authority said. To operate in the new zone, airlines need to present them with a safety case that includes the agreement of their aircraft and engine manufacturers.

UK airline Flybe is the first to achieve this and will be able to use the new zone this afternoon.