Air traffic controllers in 11 countries will take industrial action on Wednesday over an EU plan to create an integrated European airspace.
French air traffic controllers started a three-day strike on Tuesday against the proposal.
The European Commission says it will slash pollution, boost capacity, and cut costs.
Unions fear poor working conditions and job losses.
“Controllers have taken action because of the fact that the Commission is basing its logic simply on cost-cutting,” said Philippe Biol of France’s SNCTA union.
The so-called Single European Sky has also faced resistance from members states, which fear losing control of national air traffic bodies and the revenues they bring.
Siim Kallas, the European transport commissioner said: “If we leave things as there are, we will be confronted with heavy congestions, and chaos in our airspace. On the ground, the airports will be so crowded that there will be two million flights unable to take off or land.”
EU nations missed a December 4th deadline to shrink the 27 airspace area into nine blocs and now the Commission is threatening fines and legal action.