LONDON (AP) — Ex-Foreign Secretary Boris Johnson has branded British Prime Minister Theresa May’s plan for leaving the European Union “deranged,” fueling tensions as the ruling Conservative Party gathers for its annual conference.

Johnson told the Sunday Times that May’s Brexit plan won’t work, particularly proposals that would require Britain and the EU to collect each other’s tariffs.

Johnson says it’s “entirely preposterous. The idea that we could ask customs officers in Dubrovnik and Santander to charge British-only tariffs is deranged, and nobody thinks it can work.”

May’s plan would keep Britain in the EU’s single market for goods, while letting the country write its own rules on services and strike free-trade deals with third parties. EU leaders have rejected that idea, saying the U.K. wants the benefits of membership without accepting its responsibilities.