Portuguese stevedoring unions have started a strike lasting nearly a month.
Workers began refusing overtime on 10 September and will continue this until 8 October, Icelandic container line Samskip said.
The only full day of strike action is set for 0800 hours on 20 September to 0800 on 21 September.
Samskip said: "This strike will unfortunately affect the normal activities in the ports of Leixoes and Lisbon, mainly vessels' arrivals and departures.
