Could the UK withhold the £38bn Brexit divorce bill – more properly called the financial settlement – it has agreed to pay the EU? And could doing so really help renegotiate the EU withdrawal agreement the House of Commons has rejected three times?

The intriguing possibility raised by prime-ministerial candidate Boris Johnson in a Daily Telegraph interview has been met in the EU with incredulity, indifference or outrage, depending on one’s views of the Brexit process. But an adviser to French president Emmanuel Macron went...