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In 1974 ministerial government was introduced to the Gilbert and Ellice Islands Colony through a change to the Constitution. In that year a general election was held; and a referendum was held in December 1974 to determine whether the Gilbert Islands and Ellice Islands should each have their own administration. As a consequence of the referendum, separation occurred in two stages. The Tuvaluan Order 1975, which took effect on 1 October 1975, recognised Tuvalu as a separate British dependency with its own government. The second stage occurred on 1 January 1976 when separate administrations were created out of the civil service of the Gilbert and Ellice Islands Colony.

What document was changed to cause a change in government?
Constitution