Four flights, including two operating from the UK, were forced to make emergency landings at European airports after bomb threats were reported.
The affected services included a LOT Polish Airlines flight from London Heathrow to Warsaw and a Ryanair flight travelling from Manchester to Murcia in southern Spain.
A SAS flight from Copenhagen to Warsaw and another service travelling from Israel to Sofia were also subjected to similar threats.
Authorities have not said whether the incidents are linked. They occurred just one day after a Wizz Air flight from London Luton to Bulgaria was threatened and escorted by a fighter jet.
The LOT Polish Airlines passenger flight left London Heathrow Airport this morning bound for Warsaw-Chopin Airport. Following the alert, it was diverted to Warsaw-Modlin, with the warning announced at 9.54am local time.
A separate bomb threat was reported on an SAS service from Copenhagen to Warsaw. The aircraft landed at Chopin Airport and was being inspected, according to a spokesperson for Warsaw-Chopin Airport, who said normal operations were continuing.
Spanish officials confirmed that a Ryanair aircraft travelling from Manchester to Murcia had also been targeted. Meanwhile, Bulgarian media reported that a flight from Israel was grounded in Sofia after a bomb threat was sent by email.
The Boeing 737 MAX 8 landed safely at Warsaw-Chopin Airport at 10.01am, while security forces worked to verify the reported threat, a Warsaw-Modlin Airport spokesperson said in a statement.
A LOT Polish Airlines flight from Heathrow was forced to land following a bomb threat
Image shows the LOT Polish Airlines plane at Warsaw Modlin airport as passengers were evacuated following its emergency landing
Video grab shows passengers being evacuated in Warsaw Chopin airport following a bomb threat on board a SAS flight
The travel hub remained closed following the alert. In a statement, the airport said: ‘The safety of passengers, crew and airport employees is currently the highest priority.’
About 118 passengers are understood to have been travelling on the LOT flight.
Reports indicated that one runway at Chopin Airport had been closed.
Warsaw is served by two airports. Chopin Airport is located within the city, while Modlin, which is used largely by budget airlines, lies about 22 miles from the capital.
SAS confirmed that its flight from Copenhagen had landed safely and that all passengers had left the aircraft.
The Ryanair aircraft was carrying 121 people, including crew, according to Francisco Lucas, the Spanish government’s delegate in the Murcia region.
He said the aircraft was being examined and that security procedures had been activated.
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Bulgarian media reported around midday on Tuesday that a flight had received a bomb threat by email before landing in Sofia.
Counter-terrorism teams were understood to be searching the aircraft for explosives.
A spokesperson for LOT Polish Airlines told the Daily Mail that it had been ‘notified by airport authorities of a potential security threat on board a Boeing 737 MAX 8 aircraft operating flight LO286 from London to Warsaw.’
‘The aircraft was diverted to Warsaw Modlin Airport, where it landed and subsequently taxied to a designated parking stand. Relevant authorities are assessing the situation in accordance with established security procedures,’ the airline added.
It comes just a day after a fighter jet escorted a Wizz Air passenger plane flying from London to Bulgaria following a bomb threat on board the flight.
A Hungarian Gripen fighter plane was scrambled to escort the aircraft, which was forced to make an emergency landing at Budapest airport, Hungary’s defence minister Romulusz Ruszin-Szendi said on Monday.
In a statement, the minister said the fighter jet was deployed ‘within minutes’ to escort the aircraft and ensure it landed safely.
‘Our soldiers have once again proven that they can be deployed immediately if necessary and can guarantee the security of Hungary’s airspace,’ he said.
The Sofia-bound Airbus A321 departed London Luton Airport shortly after 8.30am, but was forced to make a U-turn over Bosnia and Herzegovina following the alert.
Authorities carried out an inspection of the aircraft at Budapest’s Ferenc Liszt Airport, but Wizz Air later confirmed nothing suspicious was found.
The flight, which was meant to land at Warsaw-Chopin airport, was diverted to Warsaw Modlin
Mr Ruszin-Szendi said police carried out additional measures following the landing.
Wizz Air said: ‘Wizz Air confirms that today’s London Luton–Sofia flight was diverted to Budapest following a bomb threat received during the flight.
‘The aircraft diverted and landed safely at Budapest Liszt Ferenc International Airport in accordance with the relevant security protocols and procedures, before taxiing to a designated stand, where the relevant authorities carried out the necessary security checks on the aircraft and passengers.
‘Wizz Air is co-operating fully with the authorities. The safety of passengers and crew remains our highest priority.’
The airline said authorities would conduct ‘necessary security checks on the aircraft and passengers’.
According to flight tracking website FlightRadar24, the flight was due to land at the international airport in Sofia at around 1.25pm local time.
However, a map on the tracking website shows the A321 airbus diverting towards Hungary over Bosnia and Herzegovina.