According to newspaper Correio da Manhã, the measure will be implemented after Portugal's removal from the 'red list' of countries whose travellers are subject to quarantine in hotels in the United Kingdom [… and after the UK comes out of lockdown].
The British Government’s decision is said to be seen as a "positive step" by Algarve hoteliers; Elidérico Viegas, head of the Algarve’s largest hotel association AHETA, said Portugal will have to create a system that “allows tourists to carry out Covid-19 tests before returning home”.