Bank of England Governor Mark Carney has dismissed a claim by Boris Johnson, the front-runner in the race to become UK prime minister, that Britain can avoid the hit of European Union trade tariffs in the event of a no-deal Brexit.
Mr Carney told the BBC that leaving the EU without a transition deal should be a choice taken with "absolute clarity" about what it would mean for Britain's economy.
The central bank governor said there would be long-term as well as short-term damage for Britain's economy from a no-deal Brexit and many companies were not fully ready for such an abrupt shift.
Mr Johnson has said that world trade rules include a provision, known as Article 24 of the GATT, which permits trade to continue unchanged between two parties if they so decide.
But Mr Carney said such an arrangement applied only when a trade deal was in place or about to be in place, the BBC said.
"So... we should be clear that not having an agreement with the European Union would mean that there are tariffs, automatically, because the Europeans have to apply the same rules to us as they apply to everyone else," he said.
Mr Carney has previously warned about the economic impact of a no-deal Brexit, prompting anger among Brexit supporters.
Mr Johnson and British foreign minister Jeremy Hunt, the other contender to replace Theresa May as prime minister, have said they are prepared to take the country out of the EU without a transition deal, if necessary.