Final answer:
The Conservative Party was most openly against joining the EEC in the 1960s, mainly due to concerns over national sovereignty and special interests. Britain's application to the EEC was twice vetoed by France, with Charles de Gaulle fearing American influence. It wasn't until 1973, after de Gaulle's exit, that Britain joined the EEC.
Step-by-step explanation:
The political party most openly against joining the European Economic Community (EEC) in the 1960s was the Conservative Party. Although Britain had economic interests aligned with the EEC, its desire for special protections and exceptions, as well as the unanimous decision-making policy of the EEC which allowed France to veto its membership, made the process challenging. President Charles de Gaulle of France was particularly opposed to Britain's membership, fearing it would lead to American dominance, and cut off negotiations. In the UK, the prospect of joining the EEC was controversial, with significant opposition, notably from the Conservative Party, before Britain finally joined in 1973, following de Gaulle's departure from power. A referendum in 1975 confirmed the country's EEC membership after some contention.