Brexit supporters, including the UK's Foreign Secretary, have publicly come out against the idea of a customs partnership that would effectively see Britain collect tariffs on the EU's behalf.
The idea is thought to be the one favoured by the Prime Minister Theresa May. The EU has made it clear that any proposal put forward by her government must avoid the establishment of a hard border between the North and the Republic of Ireland.
But Foreign Secretary Boris Johnson told a British newspaper that the plan as it stands is "crazy." He's reported as saying it would open up a "whole new web of bureaucracy" and would limit the UK's ability to agree on post-Brexit trade deals.
Theresa May has reportedly told officials to do more work on the proposals.
May's decision to leave the EU's customs union, which sets tariffs for goods imported into the bloc, has become one of the main flashpoints in the Brexit debate, pitting companies and pro-EU campaigners against a group of eurosceptic lawmakers.