“Perhaps, without the EU, we would now be effectively the 51st state of the United States.”

The evidence shows that EU membership did not make the UK cease to be a sovereign state: Parliament’s legal authority remained the basis of UK law, and the Commons Library notes that EU law applied in the UK only because Parliament had voluntarily accepted those obligations. The evidence also shows that the UK-US ‘special relationship’ is long-standing, rooted in shared history and values, and had endured even as Britain’s relations with Europe developed. But none of the available sources show that leaving the EU would have made Britain effectively a US quasi-state, so the causal counterfactual is too speculative to establish from public evidence. Sources: House of Commons Library, Parliamentary sovereignty and EU renegotiation; House of Commons Library, Parliamentary sovereignty; House of Commons Foreign Affairs Committee, Second Report; House of Commons Library, UK-US bilateral relationship. (commonslibrary.parliament.uk)