UK membership of the European Communities (EC) was prompted by economic and political factors. It represented a novel constitutional departure; one that was contested. The proposal for membership created divisions between, and within, both main parties. Although both Houses voted overwhelmingly in support of the principle of the membership, the short bill to give legal effect in UK law to membership was bitterly contested, the government achieving the second reading of the bill through a vote of confidence. The bill was opposed consistently by the Labour opposition and dissident Conservative back benchers, though passage of the bill was achieved eventually, courtesy of Liberals and some abstaining Labour MPs, and without amendment. The act enabled the United Kingdom to become a member of the EC, with important consequences for the UK constitution, including creating a juridical dimension unparalleled since before the Glorious Revolution of 1688. Parliament has provided for its own legislation to be subordinate to that of the EC, while adapting to the new situation through the creation of committees to scrutinise European documents.
Lord Norton of Louth, P. (2011). Divided loyalties: The European Communities Act 1972. Parliamentary History, 30(1), 53-64. doi:10.1111/j.1750-0206.2010.00242.x