Willie Walsh, chief executive of International Airlines Group, has said a British Airways test flight over the north of England and Scotland, where the ash cloud was meant to be at its densest, on Tuesday “found nothing”.

“I think we need to understand the levels of concentration that we are talking about…the levels are absolutely tiny,” he said in an interview the BBC’s Today programme.

He said that the test proved that it is safe to fly through the cloud.

“The simple answer is we found nothing,” he added.

His announcement follows Ryanair chief executive, Michael O’Leary, sparking controversy yesterday by claiming the cloud is “non-existent and mythical and misguided invention by the UK Met Office and the Civil Aviation Authority”.

Ryanair conducted a test flight 41,000ft over Scotland, which demonstrated the UK Met Office’s “red zone” forecasts were “totally unreliable and unsupported by any evidence”.

The outspoken tycoon said Civil Aviation Authority 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. 

Despite other airlines grounding their flights, Ryanair continued to fly out of Scottish airports before being ordered by the Irish civil aviation authorities to ground services.

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.”

Unlike last year’s eruption of the Eyjafjallajökull volcano, which led to a complete closure of air space, airlines have been allowed to fly if they could make a “safety case” to the CAA that there was no risk to passengers.

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

Meanwhile the Met Office has been criticised after it emerged three separate cloud monitoring aircraft have all been unavailable.

Gianni Bisignani, the secretary general of the International Air Transport Association, 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.”