The victim, named locally as Christine, is believed to have lived at the villa in the Lagoa countryside on the road to Albandeira beach for close to two decades.
She is said to have lived there with her son Louis, who also allegedly suffers from bipolar disorder.
The victim, aged 68, was found by neighbours last Saturday afternoon, dead, on the patio outside her bedroom with a stab wound to the throat.
Neighbours went to the villa after being alerted by Louis’ “odd behaviour”, newspaper Correio da Manhã reported.
GNR police told The Portugal News the case is being treated as a suspected murder, and said the main suspect is the woman’s son. The investigation has been handed over to PJ police.
PJ police said on Wednesday the victim’s son was “arrested in collaboration with the GNR” and that he will remain in custody until a preliminary investigation is completed and a court decides the measures to apply pending trial.
According to sources, Louis has, in the past, been committed to a unit in Portugal due to his mental health disorder and released when medication proved effective.
Acquaintances close to the victim, a retired homeopath, described her as a “lovely, lovely, gentle woman; just adorable”.
Unconfirmed reports claim the mother and son had a history of heated arguments at times, and that the GNR had been called to the villa previously, including earlier that same day, “after Louis wanted to light a fire inside the villa when temperatures were around 38 degrees Celsius outside”, Correio da Manhã added. The British Embassy in Lisbon said:”We are in contact with Portuguese authorities following the death of a British woman in the Algarve, and are ready to assist her family if requested to do so.”