London, England (CNN) -- Commercial European flights will be severely disrupted as long as some levels of ash are detectable in the air, the UK Civil Aviation Authority (CAA) told CNN on Monday.
Despite growing pressure from air travel groups such as the International Air Transport Association (IATA) and major airlines like British Airways, the CAA said that all current jet engine manufacturers specify zero levels of atmospheric ash for safe flying.
The CAA added that unless jet engine manufacturers changed their operating specifications, something it added was highly unlikely in the short-to-medium term, the restrictions will continue to apply.
Addressing calls from some quarters of the European travel industry to lift or ease flight restrictions, a CAA spokesman said: "We need evidence to prove that it is safe to fly... we have evidence that ash adversely affects aircraft and at the moment the manufacturers' guidelines are zero rating with respect to ash."
The spokesman said that it would be very unlikely that NATS, the British air traffic control provider, would lift restrictions in the current circumstances.
The CAA follows guidelines set by the International Civil Aviation Organization (ICAO), a U.N. agency that talks to aircraft manufacturers, airlines, governments and other organizations to draw up its safety regulations. All European states follow ICAO rules.
ICAO established the International Airways Volcano Watch (IAVW) system, which is collating data for the current volcanic ash spreading over large parts of northern Europe.
ICAO says on its Web site: "Since volcanic ash is composed of very abrasive silica materials, it can damage the airframe and flight surfaces, clog different systems, abrade cockpit windows and flame-out jet engines constituting a serious safety hazard."
NATS decides on whether to restrict flights in designated airspace based on information provided by the UK's Met Office.
The Met Office is currently conducting numerous tests every day to measure the presence of ash in the atmosphere. It does this by using "cloud-base" recorders at various locations to fire lasers into the air to detect particles and by research test flights.
Helen Chivers, a Met Office meteorologist, told CNN: "Ash is not restricted to a particular layer of the atmosphere; some is now being detected on the surface [ground] which means that aircraft would have to fly through it to reach their destinations."
Chivers said that the Met did not provide information about the concentrations of ash, only where it was present and forecasts of how the ash clouds would move with the changing weather patterns.
Responding to airline and air travel group pressure, a NATS spokeswoman told CNN: "Our decisions are not an over-reaction, we did not make them on a whim. We are anxious to fly but these are unprecedented times that are rewriting the rules."
Some experts back NATS decision to restrict flights. Guy Gratton, head of the Facility for Airborne Atmospheric Measurements at Britain's Cranfield University, flew into the skies Thursday and saw "a really strange and complex set of layers of ash," with a layer of perfectly clear air suddenly giving way to a layer of ash, he told CNN.
If particles of ash enter a jet engine, when they come out they can solidify on turbine blades, he said.
A group of his colleagues took to the skies Sunday, and in some places saw "quite high concentrations of ash," he said. "I suspect it's going to be a few days yet" before it's safe to fly, Gratton added.
However on Monday IATA "sharply criticized" European governments for their handling of airspace rules, saying "Governments must place greater urgency and focus on how and when we can safely re-open Europe's skies."
Giovanni Bisignani, director general and CEO of IATA told CNN the disruption is costing airlines at least $200 million a day in lost revenues and that if flight restrictions continue, some small and medium-sized airlines could be put in jeopardy.
British Airways' Chief Executive Willie Walsh said: "The analysis we have done so far, alongside that from other airlines' trial flights, provides fresh evidence that the current blanket restrictions on airspace are unnecessary.
"We believe airlines are best positioned to assess all available information and determine what, if any, risk exists to aircraft, crew and passengers."
But until NATS lifts its restrictions, a decision ultimately vetoed by its regulator the CAA, no aircraft are allowed to fly in the restricted zones. The British Airways test flight from London's Heathrow to Cardiff in Wales on Sunday required special NATS permission.
Similar airspace rules govern the air traffic flows over other European countries which also follow ICAO rules and are therefore likely to follow similar decision patterns.
There are exceptions like Hangar8, a private charter airline based in Oxfordshire, England, which confirmed to CNN that its aircraft flew to Europe from the UK over the weekend. The company flew outside of the controlled airspace and against official NATS advice but claimed its flights were safe.