Thousands of passengers face travel chaos after talks aimed at avoiding industrial action by Aer Lingus pilots collapsed.
The trade union representing the staff said that with no other talks planned, the action scheduled to begin on Tuesday seemed set to go ahead.
The pilots' representatives said they were discussing their way forward after Labour Relations Commission talks with the airline broke down.
Members of the Irish Airline Pilots' Association (IALPA)/Impact trade union had voted in favour of industrial action after a dispute over rosters.
In a statement Aer Lingus confirmed the talks aimed at resolving the dispute had ended without agreement.
"The company will now consider what contingency plans can be put in place for its customers to deal with the planned industrial action by IALPA-IMPACT, due to commence on Tuesday June 7," it said.
"Customers who wish to cancel bookings for travel on June 7 and 8 or wish to change to an earlier of later date can do so via aerlingus.com.
"We apologise to our customers for the uncertainty and inconvenience this causes."
While Belfast flights, plus regional flights within the UK, should not be affected, other routes faced major disruption.
Last week Christoph Mueller, Aer Lingus chief executive officer, said failure to agree rostering would "destroy our ability to operate a proper schedule".