Britain's proposal for a customs partnership with the European Union after Brexit is "crazy", foreign minister Boris Johnson says, deepening divisions that are challenging Prime Minister Theresa May.
Johnson, in Washington to try to win over US President Donald Trump to the Iran nuclear deal, spoke to the Daily Mail newspaper to add his voice to an increasingly bad-tempered debate over Britain's future customs arrangements.
Brexit campaigners have come out against what some say is May's preferred option for future ties with the EU, throwing their weight behind another proposal - one dependent on technology to make any new customs facilities as invisible as possible.
After business minister Greg Clark again made the case for the customs partnership, which would effectively see Britain collect tariffs for the EU, Johnson responded by saying it was a "crazy system".
"It's totally untried and would make it very, very difficult to do free trade deals," Johnson said in an interview published in Tuesday's Daily Mail.
"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."
He said that was "not taking back control" of Britain's trade policy, laws, borders or money, repeating his main arguments used before the referendum on EU membership in 2016.
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 in Britain, pitting companies and pro-EU campaigners against a vocal group of hardline eurosceptic lawmakers.
The prime minister has to tread a fine line, mindful of fears that any new customs infrastructure at the border between Northern Ireland and EU member Ireland could reignite sectarian violence that has not been seen for decades.
A meeting of her so-called Brexit war committee failed to come up with an answer to the future customs arrangements last week and ministers have said that they will take their time to make sure they agree on the "right" solution.
