UK NATIONALS' EU RIGHTS AFTER BREXIT - Our view
6 November 2018
Until Brexit becomes fully effective - which will depend on a lot of yet unknown factors and, if not reversed, most probably subject to a relatively long "transition period" - an UK citizen keeps full rights as an EU citizen. Then, even if he ends up losing such rights, if at the time he is a resident of Portugal, he will in all probability keep full rights of residence in Portugal as though he were an EU citizen, and have, so to speak, a "quasi-EU" status, which has already been more or less guaranteed by the EU and the UK.
We cannot, of course, guess the future, but we feel confident that UK citizens who move to Portugal before March 31st 2019 - along with the many thousands of other Portugal-resident Brits - won't face any adverse consequences. It is therefore highly unlikely that, for example, he will be subject to any minimum stay requirements.