The Queen of England ceased to be the head of state of Nigeria at
Answer Details
The Queen of England was the head of state of Nigeria before the country gained independence from Britain on October 1, 1960. However, Nigeria remained a member of the British Commonwealth with the Queen as its symbolic head. This changed when Nigeria adopted a new Republican Constitution in October 1963, which abolished the monarchy and established Nigeria as a republic with an elected President as its head of state. Therefore, the answer is "the adoption of the Republican Constitution in October 1963".