Portsea Island is a small, flat and low-lying island just off the south coast of England. The island is totally within, and contains a large proportion of, the city of Portsmouth. The third largest by population of any island in the British Isles after the mainlands of Great Britain and Ireland, it has the highest population density. To the east lies Langstone Harbour, and to the west is Portsmouth Harbour. To the south, it faces into the Spithead area of the wider Solent. A narrow channel, known as Portsbridge Creek, separates the island from the mainland.