Europe's use it or lose it airport slot rule has not created issues for airlines during the COVID-19 pandemic nor is there any evidence of carriers operating ghost flights because of the rule, a senior European Commission official said.

"From our perspective, it is an unnecessary fuss. We actually have no evidence from any airline including Lufthansa on any amount of empty flights being operated," the official told reporters.

"Travel has been relatively unrestricted. Net bookings is still quite stable," the official said.

"For the current winter scheduling season, the Eurocontrol air traffic has been so far in the range of 73-78% of 2019 and the annual 2022 air traffic is forecast to be at 88% of 2019 levels," Commission spokesperson Stefan De Keersmaecker told reporters.

Under European Union airport rules, airlines must use at least 80% of their take-off and landing slots in order to keep them for the following year.

The EU suspended those rules at the start of the Covid-19 crisis, but has started partially restoring them, rekindling concerns over empty flights as the pandemic continues.

The EU executive's comments came a day after a spat between Lufthansa and Ryanair over the rule, which the Commission has relaxed to allow major carriers to preserve airport access during the crisis despite a sharp drop in traffic.

Budget rivals, keen to expand into once-congested airports, have criticised the move.

Lufthansa CEO Carsten Spohr told a newspaper last month that the airline still had to operate tens of thousands of additional flights in the winter to comply with the slot rule, resulting in surplus emissions.

Ryanair CEO Michael O'Leary on Wednesday hit out at Lufthansa, saying it was hampering rivals.

For this winter season, the EU says airlines will only have to use 50% of their slot rights. The figure will go up to 64% for the summer season from March to October.

Earlier, the head of European low cost carrier Wizz Air said slot rules should not be changed to protect legacy airlines.

The Wizz Air CEO said that if a company could not operate its slots they should be made available to rivals.

An easing of the "use it or lose it" rule has allowed legacy carriers to preserve airport access during the coronavirus crisis despite a sharp drop in traffic, sparking protests from low-cost rivals keen to expand into once-congested airports.

"Leave the slot rules as they used to be prior to the pandemic and the market will sort it out," Wizz Air CEO Jozsef Varadi told Reuters in an interview in Abu Dhabi.

"We would be able to operate those slots at constrained airports so why are they protected for the benefit of legacy carriers who are incapable of operating them because they are inefficient?", he added.

Varadi said easing the rules was, in a way, "distorting the market" because it protected legacy carriers struggling to fill planes from lower cost rivals that could sell all their seats.

Varadi, head of Wizz Air since its inception in 2003, said access to airports should be prioritised as a public interest.