There has been “serious criticism” of both proposed models for the UK’s customs arrangements after Brexit, a senior minister has acknowledged.
David Lidington said there were fears a customs partnership might “inhibit” the scope to do trade deals and the task was how this could be “mitigated”.
The alternative, a technology-based solution, could have “adverse” effects on the Northern Irish border, he added.
The UK is under pressure to choose one option before a key EU summit in June.
Labour said it was “deeply disturbing” that the government had yet to settle a debate about managing post-Brexit customs amid open divisions in cabinet.
The UK is due to officially leave the EU on 29 March 2019, with a transition period intended to smooth the way to the permanent new relationship, due to run until 31 December 2020.
On Tuesday, Downing Street announced it would publish its blueprint for post-Brexit UK/EU relations in the form of a White Paper before the June summit. This would set out its Brexit position and be its “most significant publication on the EU” since the 2016 referendum.
Many subjects, including aviation, financial services and fisheries are expected to feature.
Mr Lidington, a Cabinet Office minister regarded as one of Theresa May’s closest allies, told BBC Radio 4’s Today programme the document, which will extend to more than 100 pages, would “demonstrate we have thought this through” and show the scale of the UK’s ambitions.
Pressed on the lack of a decision about what would replace the UK’s current membership of the customs union, which allows for tariff-free trading between members, Mr Lidington suggested both options had drawbacks which needed to be addressed.
“There was serious criticism made about the technical detail of both of the models that are on the table,” he told Radio 4’s Today.
“For example, on the new customs partnership idea, the questions are around to what extent does this inhibit an independent trade policy for the future and can we mitigate those impacts.
“The other one is does it affect Northern Ireland in an adverse way and can we mitigate the impact of that?
“What any government does is that when we are putting forward internally some ideas about relationships with other countries, new treaties, that we test the legal risks involved.”
The BBC’s political editor Laura Kuenssberg said several cabinet ministers had acknowledged privately there could be an extension of the UK’s customs union membership to give time for a solution to be developed.
Mr Lidington played this down, suggesting the Article 50 process – governing the terms of the UK’s withdrawal – might not allow for this.
“Not only are we not asking for a longer transition period but the EU has always been very clear you can’t use Article 50 to talk about the long-term future relationship,” he said.
Two separate groups of ministers on the Brexit sub-committee are “testing” the two customs models. The partnership arrangement – which is believed to be Theresa May’s preferred option – has been strongly criticised by Brexiteers including Foreign Secretary Boris Johnson.
In February, the government published a Brexit White Paper after pressure from Labour.
Shadow Brexit minister Paul Blomfield said ministers had “wasted months arguing amongst themselves rather than negotiating in the national interest”.
“Ministers have finally agreed to publish a White Paper on the government’s negotiating position, but they still don’t know what it will say,” he said.