The European Union and its institutions have developed gradually since their establishment, including 47 years of British membership, and grew to be of significant importance to the UK. Throughout that time Eurosceptic groups had existed, opposing aspects of the Union and its predecessors. Prime Minister Harold Wilson's pro-EC government held a referendum on continued EC membership in 1975 in which voters chose to stay within the bloc with 67.2% of the vote share, but no further referendums were held as the project grew and became "ever-closer" in the subsequent Maastricht and Lisbon treaties. As part of a campaign pledge to win votes from Eurosceptics, Prime Minister David Cameron promised to hold a referendum if his government was re-elected. His (pro-EU government) subsequently held a second referendum on continued EU membership in 2016 in which voters chose to leave the EU with 51.9% of the vote share. This led to his resignation, replacement by Theresa May, and four years of negotiations with the EU on the terms of departure and future relations. This process was both politically challenging and deeply divisive within the UK, with one deal rejected by the UK Parliament, general elections held in 2017 and 2019, and two new Prime Ministers in that time, both Conservative. Under Boris Johnson's majority government, the UK left the EU on 31 January 2020; trade deal negotiations continued to within days of the scheduled end of the transition period on 31 December 2020. The UK government postponed the implementation of import controls for goods entering the UK from the EU until 1 January 2022 in order to avoid supply issues during the COVID-19 pandemic. Custom controls only applied to British goods entering the EU during this period.