Air operators around the world last night imposed a no fly-zone over Ukraine as questions grew over why passenger jets were still flying over the war zone three months after pilots were warned to avoid it.

Flights were urgently re-routed around the region potentially putting hours onto scheduled journeys and causing delays.

However, aviation safety authorities in the United States and Europe warned pilots in April about potential risks flying in or near Ukraine airspace.

The U.S. Federal Aviation Administration issued a "special notice" regarding Ukrainian airspace advising airlines to "exercise extreme caution due to the continuing potential for instability."

International aviation agencies in April also had warned pilots and airlines to avoid the Ukrainian airspace.

Aviation experts last night said operators continued to fly across the zone because it was the quickest and therefore cheapest route for some flights.

A former head of group security at airports group BAA claimed last night suggested that cost was a factor in the decision to use Ukrainian airspace.

Norman Shanks, who is also a professor of aviation security at Coventry University, said: "Malaysia Airlines, like a number of other carriers, have been continuing to use it because it is a shorter route, which means less fuel and therefore less money.

"I expect the area will be declared a no fly-zone and aircraft will have no choice but to take a different, longer route.”

The emergency measures followed the apparent shooting down of the Malaysia Airlines plane over eastern Ukraine, killing more than 300 people.

The outrage immediately raised questions over why commercial flights were using a region where attacks on aircraft have been rife.

In the last week alone two Ukrainian military aircraft were shot down and a third was damaged by a missile.

Last night a Department for Transport spokesman said flights, including those already airborne, were being routed around the region.

Airlines got the heck out of Ukraine in hours following Thursday's crash. @Newsweek “Pilots around the world have been advised to plan routes that avoid the area by Eurocontrol, the European organisation for the safety of air navigation,” a spokesman said.

Virgin Atlantic said it had “slightly diverted” two flights, one from Heathrow to Dubai route and one on the Mumbai to Heathrow route.

Turkish Airlines said all of its flights would now avoid Ukrainian airspace.

British Airways said only one flight a day used that airspace, the Heathrow to Kiev service.

A spokesman said: “The safety and security of our customers is always our top priority.

“We are keeping those services under review, but Kiev is several hundred kilometres from the incident site.”

It is understood passenger jets continue to cross volatile regions because operators believe they fly at a high enough altitude to not be at risk of attack.

It was for these reasons that commercial airliners continued to fly over Iraq and Afghanistan during prior conflicts without much hesitation, although it has been reported that the US Federal Aviation Administration has recently told carriers to avoid the nearby region of Crimea.

The Malaysian Airliner was reportedly travelling at an altitude of 33,000 feet – a height considered by those within the industry to be completely safe.

Military jets typically fly at much lower altitudes, meaning it would be hard to misinterpret an airliner at such height for a threat, while many ground-based weapons would not reach such an altitude.

On Monday a Ukrainian military transport plane carrying eight people was hit by a missile fired from Russian territory killing two of those on board.

On Wednesday a Ukrainian Air Force fighter was also hit by a missile forcing the pilot to eject the aircraft.

Earlier that day another Su-25 was hit by a rebel missile but the pilot landed the plane successfully with relatively slight damage.

An industry source told The Telegraph: “The belief was that a plane could not be shot down at that altitude, which is why aircraft continue to fly over zones that have wars going on.”

David Kaminski-Morrow, air transport editor of Flightglobal magazine, added: “Any decision about the opening or closing of Ukrainian airspace will be a matter for the Ukrainians. It could well be that part or all of that airspace will now be closed.”

The incident involving MH17 is not the first occasion on which a civilian airliner has been shot down over a war zone.

On July 2, 1988, an Iran Air 655 Airbus was hit by the US Navy ship Vincennes at an altitude of 14,000 feet, causing the deaths of all 290 people on board, after being mistaken for a military aircraft.