Thousands were affected when security guards at Charleroi airport walked out in a row over privatisation
The BBC reported that all flights in and out of Ryanair's Belgian hub at Charleroi airport were grounded on Friday after security staff walked out in a wildcat strike.
A row over the privatisation of public sector security personnel caused the disruption, which began at dawn and spread to Liege airport in the east.
An estimated 8,000 passengers have been affected by the action so far.
Ryanair commuters would have been impacted the most as the airline uses the airport as a major European hub.
Low-cost airlines fly about 2.5 million people, mainly business travellers, to Charleroi every year.
The strike did not affect Brussels' main international airport, formerly known as Zaventem.
But Ryanair said the last-minute action called by the Ministry of Equipment Transport Union (MET) had also led to the cancellation of about 45 flights to other European destinations.
"This is the second time in the past 12 months that an illegal strike has disrupted Charleroi in this way," Peter Sherrard, head of communications at Ryanair told the BBC.
"The MET have shown contempt to travellers, but the Wallonia's regional government is to blame for not having contingency plans in place under such circumstances to prevent the whole airport grinding to a halt."
Union officials and management were due to have talks later on Friday.
Other budget airlines, including Hungarian Wizzair and On Air, were also affected, diverting flights to French airports Lille and Paris and the main Brussels airport.