On 1 January 1973, the United Kingdom and Gibraltar joined what was then known as the European Communities, under terms negotiated by the then Conservative Prime Minister Edward Heath. In accordance with British constitutional convention, specifically that of parliamentary sovereignty, accession was not subject to approval by referendum. Both the Conservative Party and the Labour Party were divided over the issue, and the European Communities Act 1972 was passed due to sufficient Labour abstentions to counteract the number of rebel Tories. In the run-up to the February 1974 general election, the Labour Party manifesto promised a referendum "on renegotiated terms", which its leader Harold Wilson hoped would end the division of his party. However, the election proved indecisive, and marked the end of Heath's premiership as Prime Minister; Wilson became PM, and called a second election later in the year. After the October 1974 general election, the Labour Party formed a minority administration and held the referendum on continued membership the following year, which was approved by 67% of voters.