The UK’s Foreign & Commonwealth Office has the following snippet on its ‘advice to travellers to Portugal,’ specifically for those with their own vehicles:
"You must not use your vehicle for any other purpose than tourism or loan it to anyone else during that time. If you intend to stay longer, you must apply to the Portuguese Customs authority to have the car legally imported. You will be fined if you leave the country without your car.”
The ‘you will be fined’ aspect may need some further explanation, so here it is:
If a Briton (or other nationality) brings in a car and uses it in Portugal for more than 183 days (6 months) then they should legally register the vehicle with the Portuguese authorities.
Should someone be stopped by the Police driving in a foreign registered car then the first thing they usually will be asked is how long they have been in Portugal. If they have been here for more than 183 days then they will be fined. However, it is also true that it is much harder for the Police to prove that someone came into Portugal when they said they did so this is up to the honesty of each individual.
If a foreign registered car is left in a public area and is reported as abandoned or the Police become aware of it, i.e. parked in a street or public car park, the Police will wait 30 days before impounding it. The PSP say that it is difficult and quite a lengthy process to fine owners of abandoned foreign vehicles so if after a substantial amount of time, the vehicle is not claimed, then it is destroyed. However, if the owner returns and wants to redeem his/her vehicle they will have to pay a hefty fine as it includes
a) An amount for not having registered the vehicle within 183 days (assuming it has been in the pound for quite some time)
b) An amount for the tow truck service
c) An amount per day that the car has been kept in the pound
So even though it is unlikely that a Briton who leaves their car in Portugal in a public space and returns to the UK without it will receive a fine, it is highly likely that if they return to Portugal to claim their vehicle that has been towed away they will have to pay a substantial fine.
This does not apply if the vehicle has been left in a private garage or parking area however, the 183 day rule still applies.