Under the United Nations Charter, all EU member states have agreed that: "In the event of a conflict between the obligation between Members of the United Nations under the present Charter and their obligations under any other international agreement, their obligations under the present Charter shall prevail." —Article 103. This would mean that the EU cannot prevent a member from leaving, if the state could prove that its membership of the EU conflicts with part of the UN Charter; similarly states are only bound to follow EU law 'so far as they are compatible with existing international arrangements' (Article 37.5, Treaty of Rome). If a state were to wish to leave, it would be up to the European Court of Justice to interpret current treaties as to the member's obligations and conditions of withdrawal .
Under the theory of state of exception, it is possible that a national government could suspend all laws in its country, effectively withdrawing from the EU. The French Constitution, for example, contains clauses that allow for its entire suspension; this could suspend the EU laws in a country too. However, this would have to be justified in an extremely exceptional circumstance.
By precedent, then, if a country wanted to withdraw from the EU it probably could, but special treaties and conditions would needed to be agreed on. This is because of pre-existing commitments that any member state would have towards the EU and its fellow members.
In 1975 the United Kingdom held a referendum in which the electorate was asked whether the UK should remain in the then European Economic Community (EEC), commonly referred to as the Common Market in the UK. The UK had joined the EEC on 1 January 1973 under the Conservative government of Edward Heath. The general election held in February 1974 was won by the Labour party, who had made a manifesto commitment to renegotiate Britain's terms of membership of the EEC and then hold a referendum on whether to remain in the EEC on the new terms.
All of the major political parties and mainstream press supported continuing membership of the EEC. However, there were significant splits within the ruling Labour party, the membership of which had voted 2:1 in favour of withdrawal at a one day party conference on 26 April 1975. Since the cabinet was split between strongly pro-Europeans and strongly anti-Europeans, the Prime Minister, Harold Wilson, made the decision, unprecedented outside coalition government, to suspend the constitutional convention of Cabinet collective responsibility and allowed ministers to publicly campaign against each other. In total, seven of the twenty-three members of the cabinet opposed EEC membership.
On 5 June 1975, the electorate were asked to vote yes or no on the question: '"Do you think the UK should stay in the European Community (Common Market)?" Every administrative county in the UK had a majority of "Yes", except the Shetland Islands and Western Isles. In line with the outcome of the vote, the United Kingdom remained within the EEC and later the EU.
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