Horsea Island was an island located off the northern shore of Portsmouth Harbour, England; gradually subsumed by reclamation, it is now connected to the mainland. Horsea falls within the city of Portsmouth and was wholly owned by the Ministry of Defence as part of the HMS Excellent shore establishment, which maintains its headquarters on Whale Island.However, in 2013 the south-eastern corner was acquired by Portsmouth City Council for housing development. Most of the area to the south-west of the lake is part of the Portsmouth Harbour SSSI, the remainder was declared a SINC in 2011.