A plane carrying von der Leyen to Plovdiv on Sunday afternoon was deprived of electronic navigation aids as it approached the city’s airport, in what three officials briefed on the incident told the FT was being treated as a Russian interference operation.

"The entire GPS system of the airport was jammed. There was undeniable interference," one of the officials said. After circling the airport for an hour, the pilot of the plane made the decision to land manually, using analog maps, they added.

The Bulgarian Air Traffic Services Authority confirmed the incident in a statement to the Financial Times.

"Since February 2022, there has been a significant increase in cases of [GPS] jamming and, recently, spoofing," they said.

"This interference disrupts the accurate reception of [GPS] signals, leading to various operational challenges for aircraft and ground systems," the statement added.

So-called GPS jamming and spoofing, which distort or prevent access to the satellite navigation system, were traditionally used by military and intelligence services to defend sensitive sites, but have been increasingly used by countries such as Russia as a means of disrupting civilian life.

Von der Leyen, tour of Eastern European countries. Visit to Romania too

Von der Leyen was flying to the central Bulgarian city to meet with the country's Prime Minister, Rosen Zhelyazkov, and to visit a munitions factory. The Commission president was on a tour of frontline EU states to discuss efforts to improve the bloc's defense preparedness in response to Russia's war in Ukraine.

The President of the European Commission will arrive in Romania later today. She will travel to Constanța where she will meet with President Nicușor Dan.

"[Russian President Vladimir] Putin has not changed and will not change. He is a predator. He can only be kept in check by strong deterrence," von der Leyen told reporters on Sunday during her visit to Bulgaria.

Von der Leyen left Plovdiv on the same plane, without incident, after the visit.