European airline pilots warned governments and safety regulators today against making "rash" decisions to allow planes to fly through volcanic ash clouds amid growing pressure from airlines across Europe to create "safe flying corridors" to get flights moving again.

Tonight the UK's air safety watchdog, the Civil Aviation Authority, announced it was reopening the UK's airspace after holding meetings with national air traffic controllers, airline executives and Lord Adonis, the transport secretary, to discuss flying through zones with small amounts of ash.

But the European Cockpit Association, told the Guardian today that any attempts to establish "safe flying corridors" through airspace where ash was present should not be rushed.

Martin Chalk, a professional pilot and president of ECA which represents 40,000 pilots across Europe, said: "I don't think there is a definitive answer to whether it is safe or not … We don't have much of the knowledge we would need to certify flight through low levels of volcanic ash. The three areas are regulation, technical solutions and training for the flight crew. ECA is concerned that such far-reaching decisions would be taken under such enormous human, political and commercial pressures."

Chalk warned that engine failures caused by the ash, a potentially lethal mix of rock and glass, could have serious consequences. "Were a plane to lose all power halfway across the Atlantic and have to ditch, tragedy is almost certain because of the temperature of the water," he added.

To add to the woes of stranded passengers, the government's emergency strategy to get them back to Britain was beginning to unravel. Gordon Brown had said there were 100 coaches waiting in Madrid to help transport passengers flying in from Asia, Africa and North and South America to continue the journey back home.

But consular officials there faced angry and weary travellers at the city's Barajas airport who discovered that the promised coach service did not yet exist. Embassy officials said they were working hard on the logistics but, for the moment, their advice was for everyone to make their own way to the Channel.

Brown's dramatic plans to send Royal Navy ships to repatriate stranded Britons were also thrown into doubt. The prime minister announced the move yesterday but defence officials told the Guardian today that neither the aircraft carrier Ark Royal nor HMS Ocean would be used to pick up passengers at Calais – not least because the commercial ferry operators seemed to be coping well, and both ships were too big to get into the Channel port. While Ark Royal will sail to an exercise in the Atlantic as previously planned, officials said Ocean was ready to make for Cherbourg if required to do so later in the week. One navy ship did join the rescue operation.

HMS Albion set sail from Santander in northern Spain with 280 British civilians and 200 troops aboard earlier today. It is due to dock in the UK tomorrow night or Thursday morning. Airport operators condemned the present "blanket approach" to banning flights. The airports claimed other European countries were already operating more flexible policies as the Iceland volcano eruption continued to paralyse air travel in and out of Britain for a sixth day.

The CAA board is expected to discuss whether there is enough meteorological and test flight data to gauge the risk in implementing safe flight corridors this afternoon.

On Monday, EU transport ministers agreed to split the continent's airspace into three areas: a no-fly zone around Iceland and the Eyjafjallajokull volcano; safe corridors; and flight paths to be opened by aviation regulators despite the presence of ash.

The Association of Airport Operators (AOA) warned that the country would remain "closed for business" unless the UK adopted measures recommended by Eurocontrol, the pan-European navigation organisation, to EU transport ministers yesterday.

The operators stoked the row over UK policy as they tried to persuade the air safety regulator, the Civil Aviation Authority, Nats and the government to move more quickly on adopting a pan-European policy of airspace closures suggested yesterday.

A proposal by Eurocontrol, the intergovernmental air traffic body, to European transport ministers suggested the implementation of a no-fly zone limited to the visible ash plume as determined by satellite images and adequate buffer areas which could be updated on a six-hour basis.

The AOA chairman, Ed Anderson, said: "The cost of ongoing disruption for the airlines and airports industry alone is £130m a day. Add to that the cost to businesses dependent on flights to move goods and people and the effect on the wider economy is critical. The government is also losing £5.5m a day in air passenger duty.

"Many thousands of UK citizens remain stranded abroad, unable to get home. Unless the UK gets its policy on airspace closure into line with the rest of Europe, Britain will remain closed for business."

A spokesman for the CAA said: "As far as I am aware, this is still being discussed in Brussels … it is not there yet." Anything decided would supersede the different policies across European states, but he pointed out Britain was nearer the volcano than other countries.

British Airways is preparing to divert about 16 incoming long-haul flights that took off from the far east, Mauritius, Mexico and the US west coast earlier today in the hope they could land at Heathrow later this evening or in the very early morning. It made contingency plans for them to land at other European airports. It cancelled another dozen planned incoming flights because of restrictions on flight crew hours .

EasyJet said any UK flights were unlikely and Ryanair cancelled northern European operations including the UK until at least Thursday at 1pm. Flights between the UK and Ireland will not resume until Friday. Aer Lingus also cancelled all flights today, while Thomson Airways will not lay on outbound flights until at least Thursday.

The National Air Traffic Services said that from 1pm to 7pm only part of Scottish airspace, including Aberdeen, Inverness and Edinburgh airports, would be available for flights in addition to airspace south to Newcastle airport.

That meant no flights after 1pm from Glasgow, which was among the airports that were able to open earlier. An update this afternoon said that from 7pm Glasgow and Teesside airports could open. However, flights are crossing UK airspace above 20,000ft.

Traffic was light even at some airports that were open. Graeme Mason, head of planning and corporate affairs at Newcastle international airport, said: "We have got about eight flights out of a normal 120 to 130, so a very, very light schedule for sure."

But there was hope that an end to the crisis may be in sight. Europe's largest short-haul airline, Ryanair, said it hoped to resume normal operations by 1pm on Friday as weather conditions improve. The Dublin-based carrier added that it expects to restore much of its European schedule by Thursday, including flights from the UK to the continent. Flights between the UK and Ireland will not be restored until Friday, however, to allow Ryanair to position its flights. The low-cost airline, which has gained notoriety for its charges, said it was waiving baggage and check-in fees "for a number of days."