Malta achieved its independence as the State of Malta on 21 September 1964 (Independence Day) after intense negotiations with the United Kingdom, led by Maltese Prime Minister George Borġ Olivier. Under its 1964 constitution, Malta initially retained Queen Elizabeth II as Queen of Malta and thus head of state, with a governor-general exercising executive authority on her behalf. In 1971, the Malta Labour Party led by Dom Mintoff won the general elections, resulting in Malta declaring itself a republic on 13 December 1974 (Republic Day) within the Commonwealth, with the president as head of state. A defence agreement was signed soon after independence, and after being re-negotiated in 1972, expired on 31 March 1979. Upon its expiry, the British base closed down and all lands formerly controlled by the British on the island were given up to the Maltese government.