LONDON (Reuters) - British Prime Minister Theresa May has told officials to do more work on future EU customs arrangements after her foreign minister attacked one of her Brexit proposals as “crazy”.

Boris Johnson’s broadside against what some say is May’s preferred option for ensuring Britain’s new border with the European Union is as frictionless as possible underlines the deep divisions over what post-Brexit ties should look like.

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.

Johnson and other Brexit supporters have come out against a proposal for a customs partnership that would effectively see Britain collect tariffs for the EU, putting pressure on the prime minister to dump the plan. [nL8N1SD0CQ]

May’s spokesman said on Tuesday work was continuing on the two proposals currently on the table - the customs partnership and highly streamlined customs arrangement that would rely on technology. The EU has dismissed both proposals.

“The prime minister asked officials to take forward that work as a priority,” her spokesman told reporters after her so-called Brexit war cabinet failed to come to an agreement on which plan to pursue at a meeting last week. [nL8N1S94RC]

“Following last week’s cabinet sub-committee meeting, it was agreed that there are unresolved issues in relation to both models and further work is needed.”

Brexit campaigners have criticized the customs partnership and believed it had been shelved until business minister Greg Clark again made the case for the proposal on Sunday. Johnson’s words were seen as a reaction to that.

“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.

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“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.”

May’s spokesman said the prime minister had full confidence in her foreign secretary.

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.