The sighting was filmed near a boat in the area and quickly gained attention online after footage was shared on social media.

Although its size can appear intimidating, the basking shark is harmless to humans. The species feeds mainly on plankton and is often seen swimming slowly near the surface in areas where food is plentiful.

Basking sharks are found in temperate waters around the world and are considered the second-largest shark species on the planet, behind the whale shark. Adults can grow to around 12 metres in length.

Sightings along the Costa da Caparica coastline are uncommon but not unusual, with the species also spotted in the area last year.