Italian, French and Bulgarian fighter jets were scrambled today as at least 14 European flights - including an Aer Lingus flight from Dublin - were hit by hoax bomb alerts for the third day this week.
Hoax bomb threats affected at least 27 flights around Europe today from Ireland to Bulgaria, forcing some to make unplanned landings and prompting fighter jets to escort or intercept others.
While no country has attributed blame for the threats, they come as many European officials are accusing Russia of masterminding sabotage and 'hybrid attacks' - which are often cheap, easily deniable and not expected to trigger an armed response.
Denmark, Poland and Romania, all affected by today's disruptions, have previously accused Moscow directly, while Germany blamed Russia for an attack on Leipzig airport in August.
Britain has blamed Russia in the past but linked Iran to a suspected plot to attack a US air base recently. Both Russia and Iran deny any involvement.
An Aer Lingus flight which left Dublin Airport at 6.24am this morning, travelling to Rome Fiumicino Airport, was diverted as a "security precaution".
French Prime Minister Sebastien Lecornu's office said fighter jets were sent to intercept and escort the Aer Lingus Flight EI402 after the pilot did not cooperate with authorities as expected.
The pilot and co-pilot were questioned as part of an investigation into what happened, a French regional prosecutor said.
An Aer Lingus spokesperson said: "Aer Lingus and its pilots fully co-operated with the French authorities regarding this matter, and the engagement of the pilots with the authorities has now concluded. Aer Lingus' priority at all times is the safety of our passengers."
There were 151 passengers and six crew on board the flight, which landed at Evreux airbase, according to Mr Lecornu's office.
It added those on board were moved to a safe place and a bomb unit attended the scene.
Meanwhile, a flight from Cork was delayed 90 minutes after the airport received a threat, airport operator daa said.
Eric Schouten, head of the aviation security group Dyami, said most of the countries affected supported Ukraine in its war against Russia's invasion.
"From a geopolitical view, I'm thinking, you always have to think about the scenario of hybrid warfare, where a country in this way - today we are looking at Russia, right? - is attacking us on multiple fronts, from railways to supermarkets to blood banks, most recently. The energy sector, the internet sector, and the aviation sector are among those," he said.
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Bulgaria's air force said its fighter jets had escorted three airliners to the ground. Ryanair planes from Copenhagen and Madrid to Sofia landed as planned while a flight from London to Sofia was diverted to Plovdiv, Bulgaria's second city.
Spanish authorities received three emailed bomb threats in under 20 minutes involving two Air Europa departures from Madrid to Paris and four inbound Iberia flights from Rome and Milan, Spain's Guardia Civil said.
Several aircraft in Madrid were halted or returned to their stands and passengers were disembarked while planes were inspected.
In Warsaw, a LOT flight to Paris was stopped as it taxied towards take-off and passengers were transferred to another aircraft.
Two planes were searched at Copenhagen airport in Denmark, police said.
Italian media reported that a Wizz Air flight from Milan to Warsaw had been diverted to Vienna.
Two Easyjet flights from Britain, one flight from Ireland and one from Greece landed as scheduled in Prague after receiving bomb threats, as did a Finnair flight from Helsinki to Vienna, a Polish LOT flight from Warsaw to Brussels, a Hungarian Wizz Air flight from Milan to Bucharest, and an Air Serbia flight from Belgrade, also to Bucharest.
The Italian news agency ANSA said two planes had landed as planned in Milan, while a person familiar with the matter said Italy's air force escorted a Finnair plane coming from Helsinki to its scheduled landing in Rome.
The operator of Rome's Fiumicino Airport said there had been no disruption to services.
Joji Waites, director of flight safety for the British Airline Pilots' Association, said it was industry practice that airlines did not act on threats unless they had been rated credible by assessors.
Eliran Almog, CEO of aviation cybersecurity firm Cyviation, said the hoaxes could serve both to disrupt operations and to observe how airlines, airports and authorities respond.
"These aviation threats are also occurring in the same period as cyber attacks on supermarket refrigeration systems across Europe," Mr Almog said.
"Different essential services are experiencing disruption that carries economic costs and can undermine public confidence."
On Tuesday, at least seven flights and a cruise ship received hoax threats.
This week, unidentified hackers for the second time shutdown fridges in supermarkets in several European countries, including in Ireland, and also affecting French blood banks.
Graeme McQueen of Dublin and Cork airport operator daa said that email threats about bombs were "very regrettable and very disconcerting for the travelling public" but it is "business as usual" for passengers.
"We want to reassure the Dublin and Cork airports are running fine today," he said, adding that anyone with travel plans should proceed as normal.
He added that all threats are fully investigated, as safety is number one and any threat is escalated to the appropriate authorities and fully checked out.
He said it was his understanding that issues arose after an anonymous email was sent about threats to publicly available email addresses such as customer services.
He said that these things crop up from time to time, and those investigating them are skilled in this area and can tell quickly if the threat is real or not, adding that this will continue.
He said that passengers should be aware that there will be no change to security rules, adding that passengers can trust that safety is the number one issue for daa.
The Irish Air Line Pilots' Association (IALPA) said pilots are "highly trained professionals, equipped to manage complex and high-pressure situations, including potential security threats," and whose primary responsibility "remains the safety and security of all passengers and crew".
It said the Aer Lingus incident highlights that any "abnormal or unexpected event increases the workload in the cockpit, placing additional demands on pilots working in an already high-pressure environment".