The action will get under way on 31 May and run through to 28 June. The Foreign Office has warned travellers they could face delays as a result.

Portugal is among only a handful of recognisable destinations on the UK government’s green list; thousands of British holidaymakers have already headed for the Algarve coast and Madeira over the past week after Portugal lifted its ban on non-essential travel on 17 May.

Action is planned at Lisbon airport between 5am and 9am from 1-15 June, and between 9am and midday at Faro over the same period.

Further action is planned at Madeira airport on 31 May, 7 June, 14 June, 21 June and 28 June, again between 9am and midday.

There is currently no action planned at Porto airport.

The FCDO updated its travel advice for Portugal on Thursday afternoon (27 May).

"Planned strike action by Portuguese immigration officers from 31 May to 28 June may cause delays," it said.

"Days and times of strikes vary according to airport location.

"If you are travelling during this period, check with your airline whether you will be affected and allow more time to clear border control when entering and exiting Portugal."