It comes after Portuguese immigration officers announced a series of walkouts to take place from 6-10 April.
According to local news agency Lusa, the five days of industrial action could cause delays at airports.
The UK Foreign Office, which updated its travel advisory to the country, said: "If you are travelling over this period, allow additional time to clear border control when entering and exiting Portugal.
"Check for announcements and follow the guidance provided by your airline or tour operator."
