Airlines selling tickets in Europe cannot add extra post-purchase fees when fuel costs surge, the European Union said on Friday, as the region's aviation safety agency warned potential shortages could force a switch to jet fuel from the United States that requires stricter safety controls.

In a guidance note to the transport sector, the European Commission barred any post-purchase ticket price increases, which is a practice the Spanish low-cost airline Volotea has applied in recent weeks.

The commission stated: "Anyone selling air tickets must always display the final price the passenger will pay. This includes all unavoidable and foreseeable taxes, fees, and charges. Adding a fuel surcharge to a ticket after it has been bought cannot be justified."

EU spokeswoman Anna-Kaisa Itkonen told reporters in Brussels that any changes post-booking can "raise issues under the EU's unfair commercial practices".

The EU guidance published on Friday said that "any retroactive change of the price is excluded".

"Airlines may not include terms and conditions which would allow them to increase the price of the ticket above what is advertised at the time of purchase simply because fuel was more expensive than they had accounted for," the commission said.

Volotea is under investigation in France after charging customers fuel surcharges, citing energy shocks related to the ongoing conflict in the Middle East.

Gilles Gosselin, the airline's country manager for France, defended the measure, telling Agence France-Presse: "The legality of our system has been confirmed by three independent law firms specializing in air transport and consumer law. The measure is transparent, it is temporary, and it works both ways (up or down)."

Amid potential jet-fuel shortfalls, the EU's Aviation Safety Agency, or EASA, cleared the way on Friday for using Jet A, a US-produced aviation fuel not currently used in Europe, except on return flights from the US for technical reasons.

But it warned airports and airlines across Europe that the switch would necessitate tighter safety measures, reported Euronews.

Jet A-1, the prevalent fuel grade in Europe, Africa, Australia, and much of Asia, has a lower freezing point than Jet A, which is more commonly used in the US and Canada.

The agency set conditions on the use of Jet A, warning that introducing it into a system long geared to Jet A-1 could pose "operational" risks when both fuels are in use.

The EASA's guidelines caution against the simplistic notion that "jet fuel is jet fuel", noting that limited training or unclear fuel-grade labeling can lead pilots and ground crews to make unsafe operational assumptions.

The aviation agency said: "A potential introduction of Jet A in Europe or in other parts of the world would not generate safety concerns, provided that its introduction is properly managed."

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