Portsea Island is a flat and low-lying natural island 24.5 square kilometres (9.5 sq mi; 6,100 acres) in area, just off the southern coast of Hampshire in England. Portsea Island contains the majority of the city of Portsmouth. Portsea Island has the third-largest population of all the islands in the British Isles after the mainlands of Great Britain and Ireland; it also has the highest population density of any British Isle, and Portsmouth has the highest population density of any city in the UK outside of London.