A series of fake bomb threats disrupted at least 25 flights across Europe on Thursday, with some aircraft forced to make emergency landings. A total of at least 33 flights have been affected this week, following similar threats on Tuesday and Monday.

The threats concerned flights between Italy, Ireland, the United Kingdom, Spain, Bulgaria, Romania, Poland, Finland, Austria, Denmark, Belgium, Portugal, France, Greece, Serbia and the Czech Republic. Among the affected carriers were Ryanair, Aer Lingus, Wizz Air, Finnair, easyJet, Aegean Airlines, Iberia, Sky Express, Air Serbia and LOT Polish Airlines. All aircraft landed safely, and searches found nothing suspicious.

Heathrow and Birmingham airports received threats overnight. With the help of specialist investigators from the aviation command of London’s Metropolitan Police, it was determined that these threats were false. The day of fake reports followed similar threats on Tuesday, when seven aircraft in Europe were searched or diverted, and after Monday, when a flight from the United Kingdom to Bulgaria was diverted due to a threat that proved unfounded.

Investigators stated on Thursday that there is still no evidence that the fake threats are being organised by a hostile terrorist group or a state actor such as Russia or Iran. The International Air Transport Association (IATA) condemned the disruptions and called on authorities to determine the source of the threats. Nick Careen, IATA’s senior vice president for operations, safety and security, said: “Safety is the number one priority for the industry and we take all threats seriously.”

European Commissioner for Transport Apostolos Tzitzikostas said it was too early to know whether the incidents were linked, but that all threats against civilians were unacceptable. Bomb threats sent by email to airports in Rome and Sofia were signed as “fighters of the Male State”, according to sources at Aeroporti di Roma, which manages two Rome airports, and a text seen by Agence France-Presse.

The Male State is a Russian far-right, ultranationalist and misogynist movement founded in 2016 by Vladislav Pozdnyakov. On his Telegram channel, Pozdnyakov did not comment on or claim responsibility for the threats, and is known as a prominent advocate of the war in Ukraine. The emails allegedly stated: “We, the fighters of the Male State, will write history with our actions today. Your feminist regime will remember this forever. The explosion will happen in 60 minutes. Today your sky will be coloured red.”

A spokesperson for Cork Airport confirmed to the Guardian that the airport received an anonymous phone threat on Thursday morning, causing one flight to be delayed. The delayed flight was a Ryanair service from London Stansted. A separate Ryanair flight from Pescara in Italy to Brussels-Charleroi was inspected after the word “bomb” was found on the toilet walls of the aircraft.

The affected flights had knock-on effects, but they represented only a small part of the 27,400 flights scheduled in Europe on Thursday, according to Eurocontrol. The pilot of an Aer Lingus flight from Dublin to Rome, which was diverted to Évreux, was questioned by French authorities because he initially refused to land at a military base, according to French media reports.

The disruptions followed cyberattacks this week on supermarket cooling systems. A Kremlin spokesperson said on Thursday that it was “absurd” to blame Vladimir Putin’s administration for the hacking of fridges. Dmitry Peskov told journalists: “We have such unfounded accusations against our country, which implicate us in everything.”

In Rome, Italian Defence Minister Guido Crosetto said: “It seems to me that every day, from fridges to blood supplies in France, to these false alarms, there is someone interested in provoking tensions.” When asked if he knew who was behind the fake threats, Crosetto replied: “This is called ‘war below the threshold’ because it is below the threshold of attribution. It is done deliberately so that no one can attribute blame.”

Many of the affected flights were operated by low-cost carriers, but there is little noticeable pattern to the incidents. Aviation experts say that fake threats happen every day, but that such a number of credible threats, even if they are malicious fake reports, points to coordinated action.

Captain Martin Drake, a former pilot and technical advisor to the British Airline Pilots' Association, said that airlines have safety teams that assess every threat on a traffic light scale: green for continuing the flight or red for landing, with the option of yellow only for flights that have not yet taken off, allowing for more thorough checks and searches of the aircraft before clearance for flight. “If you want to deliberately disrupt a flight, you have to do it while it is in the air,” Drake said.