A six-day strike by one of France’s largest air traffic controllers’ unions, the UNSA-ICNA, is set to cause major disruption to flights throughout Europe.

Starting on June 24, the strike coincides with the beginning of the European summer holiday season.

The French CAA estimates that 20% of flights could be canceled, although some airlines believe the figure could be as high as 50%, based on similar action last year.

Ryanair tweeted that it canceled 168 flights Tuesday.

Air traffic controllers are protesting over Single European Sky (SES) reforms that aim to reorganize European airspace into functional airspace blocks (FABs) based on traffic flows, rather than national borders. They claim the changes will threaten the “necessary performance and modernization needed to ensure an efficient air navigation service in France.”

IATA DG and CEO Tony Tyler condemned the strike action, describing the unions as being “bent on stopping progress” and the strike’s timing as “malicious.”

“There are more borders in the skies over Europe than exist on land and that comes at a great cost,” he said. “[SES] can be done without a single controller losing his or her job. Who could be against that? This strike is totally unjustified.”

Eurocontrol estimates that continued delays to full SES implementation resulted in 70 million minutes of delays for aircraft in 2012, equivalent to 133 aircraft being grounded for a year.

Ryanair also condemned the strike and called on the European Commission (EC) to remove the right to strike from Europe’s air traffic controllers, who are “attempting to blackmail ordinary consumers.” The low-cost carrier urged the French government and the EC to intervene to prevent further cancellations and delays on Wednesday.

“It is high time the EU Commission removed these air traffic controllers’ right to strike, in the same way as ATC in the US, and many of Europe’s armies and police forces, are prohibited from striking by law, to stop Europe being held to ransom ... every summer,” Ryanair chief marketing officer Kenny Jacobs said.