Irish holidaymakers jetting off to the Algarve and other popular destinations in Portugal are facing travel chaos.
Security staff at all Portuguese airports and ports have indicated they will strike over the busy Easter period.
Airlines have advised passengers to give themselves lots of time to clear security as they return home, with long queues and delays expected.
The Union of Aviation Workers and Airports (Sitava) said security staff employed by the private companies Prosegur and Securitas, which are responsible for security checks, have announced strike action that will last from 4am local time yesterday until Tuesday.
Although a skeleton staff will be on hand, staff will be striking for two hours at the start of each shift.
ANA, the national airport management company in Portugal, has advised all passengers travelling to the country over the next five days to expect delays and long queues at security checks.
It advised passengers to keep their hand luggage to a minimum.
A spokesman for Aer Lingus said the strike will affect hundreds of Irish passengers who are heading for a holiday during the start of the busy summer season this weekend.
The airline has told passengers to allow lots of time to clear security and to head directly to the security area after checking in.
To speed up the process, Aer Lingus has waived its normal fee to check-in cabin luggage and is urging passengers to check-in all luggage.
"We have advised our guests to arrive early at Lisbon Airport and to make their way to the security screening point immediately after checking-in to ensure that they reach the gate on time for their flights," the spokesman said.
"We are inviting our guests to check-in their cabin baggage free-of-charge at the airport check-in desk in order to speed up their journey through security."
A spokesman for Ryanair, which flies to Faro and Lisbon, said passengers should arrive at least three hours ahead of their departure time to ensure they clear security in time.
The strike affects all of the country's airports and ports. However, the largest airports in Lisbon, Porto, Faro, the Azores and Madeira will be the worst hit as holidaymakers from across Europe descend on Portugal over Easter.