Boris Johnson has attacked Theresa May’s “crazy” post-Brexit customs rules.

The foreign secretary said in Washington that the customs partnership, where the UK would collect tariffs on behalf the EU, was unacceptable.

Johnson said to the Daily Mail that the proposed solution was “totally untried and would make it very, very difficult to do free trade deals”.

“If you have the new customs partnership, you have a crazy system whereby you end up collecting the tariffs on behalf of the EU at the UK frontier. If the EU decides to impose punitive tariffs on something the UK wants to bring in cheaply there’s nothing you can do.”

The comments made have been the strongest yet in suggesting that Johnson may resign, should the prime minister push ahead with the current plans.

The other option being considered, preferred by Johnson and other pro-Brexit minsters is the maximum facilitation proposal, which will keep border checks to a minimum.

According to the foreign secretary, the partnership model would not appease Brexiteers or the referendum result. 

“That’s not taking back control of your trade policy, it’s not taking back control of your laws, it’s not taking back control of your borders and it’s actually not taking back control of your money either, because tariffs would get paid centrally back to Brussels.”

“It only solves the Northern Ireland border question if you force companies to prove that an imported tariff-reduced good has been consumed in the UK and if you insist on complete regulatory alignment with the EU rule book,” he added.

“Otherwise if Britain chose to vary its laws in any way at all on goods and agrifood, then logically you would need checks at the border.”

The UK will need to confirm a solution to the customs issue at a summit held at the end of next month.