Britain would be “humiliated” and “irrelevant” on the world stage if it were to remain in a customs union with Europe after Brexit, the Australian high commissioner said last night.

Alexander Downer told an audience in London that he had “been surprised that there is any debate at all” about Britain remaining in a customs union, which would leave Brussels in charge of trade negotiations without British input.

“For us in the outside world, Britain would become, at least in economic terms, irrelevant to international diplomacy,” Mr Downer said. “You would be better off remaining within the EU or being completely out of it in every way than leaving yourselves in such a position of weakness and irrelevancy. It would be humiliating for a