Willie Walsh, chief executive of International Airlines Group, said BA had conducted a 45-minute test flight at different altitudes over the north of England, Newcastle, Glasgow and Edinburgh, where the ash cloud was meant to be at its densest, on Tuesday.

Disclosing the results of tests carried out on the aircraft after the test run, he told the BBC's Today program: "The simple answer is we found nothing."

Pledging to make a case to the Civil Aviation Authority that the test proves it is safe to fly through the cloud, he added: "I think we need to understand the levels of concentration that we are talking about...the levels are absolutely tiny."

His remarks came after Michael O'Leary, the outspoken head of Ryanair, described the ash cloud as "mythical" after the airline operated a similar test flight across air space with the highest ash densities.

Rounding on the Civil Aviation Authority, he said its officials should "take their finger out of their incompetent bureaucratic backsides and allow the aircraft back into the skies over Scotland".

More than 1600 flights were grounded and thousands of passengers left stranded after the cloud of dense ash settled over much of Scotland on Tuesday.

As the cloud started drifting eastwards towards northern Europe, the war of words intensified in Britain over the wave of cancellations triggered by the eruption of the Grimsvötn volcano in Iceland.

Giovanni Bisignani, one of the most senior figures in the aviation industry, condemned the failure of the Met Office to procure a plane capable of inspecting the cloud at close quarters as "astonishing and unacceptable".

Privately other airlines have also voiced doubts over the Government and Civil Aviation Authority's approach, even though the disruption has been nowhere as bad as during last year's ash crisis.

While last year's eruption of the Eyjafjallajökull volcano in April last year lead to the complete closure of air space, airlines have been allowed to fly if they could make a "safety case" to the Civil Aviation Authority that there was no risk to passengers.

The majority of airlines have, in consultation with plane and engine manufacturers, put a safety case to allow flights to take place in moderate concentration of ash.

But Ryanair appeared to break ranks by running a flight from Glasgow Prestwick airport to Inverness, Abderdeen and Edinburgh, plotting a route which it said was through the "red zone" of concentrated ash.

The carrier insisted that no engine damage had been sustained, before describing the "red zone" as "a misguided invention by the UK Met. Office and the Civil Aviation Authority."

Michael O'Leary added: "This is just a bureaucratic balls up and now they are trying to cover their backsides."

While other airlines grounded their flights, Ryanair continued to board passengers at Scottish airports before being forced to cancel services on the instructions of the Irish civil aviation authorities.

The Civil Aviation Authority insisted on Tuesday night that the Ryanair flight had not gone through the zone with the highest ash concentration.

Philip Hammond, the Transport Secretary, said: "I do not think Michael O'Leary should take the law into his own hands and say I think it's safe to fly. It's not a responsible course of action.

"Neither I nor the CAA are going to be bullied by an airline operator."

Meanwhile the Met Office, which was heavily criticized over last year's ash crisis, came under fire again after it emerged three separate cloud monitoring aircraft have all been unavailable.

The absence of the planes triggered fury among airlines who voiced their disbelief that no money was put aside to procure a substitute aircraft capable of carrying vital testing equipment.

While other measurements can be taken from the ground and using radar, experts questioned whether they would be as accurate and useful as those provided by an aircraft flying through the cloud.

One aircraft, a BAE 146, which the Met Office and the Natural Environment Research Council lease, is about to fly the Sahara on a long-arranged research project. The same plane was unavailable for nearly a week during last year's volcanic ash crisis.

The second, a new Cessna 421 which is being made available for the Met Office, will not be ready until late next month.

Meanwhile the third, a Dornier 228, which was used last year to carry out a rudimentary inspection of the ash, was damaged by the exercise and is now out of commission.

Mr Hammond told the Daily Telegraph that an Irish inspection aircraft could be pressed into service today.

But Mr Bisignani, the secretary general of the International Air Transport Association, the body representing the world's leading scheduled airlines, voiced his anger in a letter to Philip Hammond, the Transport Secretary.

"It is astonishing and unacceptable that Her Majesty's government cashes £3.5 billion a year in Air Passenger Duty but is incapable of using a small portion of that revenue top purchase another Cessna to use as a backup aircraft. I ask please that you ensure that all possible efforts are made to get the existing aircraft operational in the shortest possible time."

Mr Bisignani also said passengers were being hit by "the patchwork of inconsistent state decisions on airspace management"

Another senior aviation source also questioned how the latest ash crisis was being handled, even though the chaos was not on the scale which followed the eruption of Eyjafjallajökull.

"'You have to question what is the greater priority, a long-term commitment to a project in Africa or the imperative of getting a plane up into the cloud," said an aviation industry source.

"Even though the Saharan work is worthwhile, they should be scrambling that for the time being and that plane should be heading towards Scottish air space and not North Africa."

A Department for Transport spokesman sought to play down the importance of the inspection aircraft.

"The UK is taking detailed readings to monitor the ash cloud from a number of different instruments including radar and lidar sites, improved satellite data and weather balloons. Test aircraft only provide a snapshot in a localized area, whilst the Met Office model looks at the bigger picture, and the accuracy of this model is considered to be high.

"However, as part of our ongoing efforts to reduce the impact of the ash cloud, we are looking to make a specialized test aircraft in the future."

A five day Met Office forecast suggests that the whole of Britain could be in the "red zone" on Saturday with dense ash at heights of above 35,000 feet.

The accuracy of the forecast depends on both the wind direction and how active the volcano - which appears to be subsiding - remains over the next 48 hours.

Should the Met Office forecast prove accurate, there is likely to be disruption with airlines having to re-route to avoid flying through the high density ash, although widespread cancellations are currently not expected.