Finnish MEP Laura Huhtasaari argued "inspirational" Britain was "striving for more freedom and democracy" as negotiations over trade begun earlier this year.
She told Parliament that once the UK left, the "EU will never be the same again", because Britain will "reach its full potential, and that will cause criticism against the EU in the remaining 27 member states".
Mrs Huhtasaari added: “People are going to realise that the current technocratic EU is just holding us back.
“A truly sovereign state is able to decide on its own laws, taxes and borders.
“As a member of the EU, a nation state relinquishes these rights, which creates a severe democratic deficit.”