It has been reported that the Hong Kong authorities has classified the UK as an "extremely high-risk" country.
The ban, which has been made in response to the rise in UK cases, particularly the Delta variant, will come into effect on 1 July.
The UK as a whole is experiencing the highest number of daily cases in Europe, most of which are linked to the Delta variant.
It is understood Hong Kong reported its first case of the Delta variant last week.
The flight ban comes amid political tensions between the West and Hong Kong over the city’s brutal crackdown on dissent and opposition media.
Those arriving in Portugal have to quarantine “at home or a place indicated by the health authorities” unless they can prove that they received their second vaccine dose a fortnight before arrival.
These quarantine rules only apply to those travelling to mainland Portugal – not Madeira.
Malta has also put restrictions on UK travellers with authorities announcing that only "fully-vaccinated travellers" will be allowed to enter the country from the UK – children under 12 will be permitted if they are accompanied by parents or guardians who have had both doses.
In Spain, UK travellers need to prove they are fully vaccinated, or provide a negative PCR test on arrival.