Liz Truss, the Trade Secretary, has warned that Boris Johnson's Brexit border plans could result in smuggling and the breaking of international rules in a leaked letter.
The Government announced last month that full border controls will not be applied on goods until July 2021, despite Britain leaving EU trading and customs rules at the end of 2020. Business groups fear the delay risks a "disaster" for firms trading with the EU.
In an explosive letter to Cabinet ministers on Wednesday, Ms Truss warned that the new regime risks the UK's international credibility.
The leaking of the letter to Business Insider has raised suspicions that she may be moved from her post in a mooted summer reshuffle.
In the private correspondence to Rishi Sunak, the Chancellor, and the Cabinet Office minister, Michael Gove, Ms Truss highlighted four "key areas of concerns" about the Government's border plans, which are due to be published on July 13.
Saying that the proposals would create a series of logistical, political and reputational risks for the Government, the letter demands "assurances that we are able to deliver full control at ports by July 2021 and that plans are in place from January to mitigate the risk of goods being circumvented from ports implementing full controls".
Suggesting that a lack of preparedness could lead to smuggling from the EU if UK ports are not ready to carry out checks, she said the UK could "be vulnerable to WTO challenge".
This is because Britain plans to temporarily give the EU preferential treatment, which could breach WTO rules if there is no UK-EU free trade agreement in place.
Angela Merkel, the German chancellor, said on Wednesday that the EU should be prepared for a no trade deal Brexit, a day after Mr Johnson told her Britain was "ready" to walk away without an agreement.
Ms Truss also expressed concerns over the union if EU tariffs are applied to all goods heading to Northern Ireland by "default."
"I understand that the digital delivery of the dual tariff system (both EU and UK tariff) in Northern Ireland is a high risk and that HMRC are planning to apply the EU tariff as a default to all imports in NI on 1 January 2021," she wrote.
She also said she was worried that it would anger unionists in the province, telling Mr Gove and Mr Sunak: "This is very concerning, as this may call into question NI's place in the UK customs territory."
The letter also appeared to confirm that a previous UK Government plan to waive customs declarations on exports to the EU had been dropped.
"I am pleased to hear that following the XO [Exit Operations] meeting last Friday, it was decided that the temporary waiving of export declarations will not be included in the publication," Ms Truss wrote.
She signed off by telling her colleagues that "we need to ensure that the UK border is effective and compliant with international rules, maintaining our credibility with trading partners, the WTO and with business".
The UK trade department needs a "clear view of operational plans, timescales and risks going forward," she said.
On Wednesday, the Environment Secretary said Britain could forgo annual negotiations with the EU over fishing opportunities for some types of fish.
In the latest sign a deal over British waters could be edging closer, George Eustice told a House of Lords Select Committee: "It's not uncommon for there to be a multi-annual agreement, but not necessarily on all stocks [...] it can happen on individual stocks."
The UK has demanded a Norway-style fishing deal with annual negotiations over fishing opportunities. The EU appears ready to drop its demand that its boats retain status quo access to UK waters provided a deal is negotiated for the long term rather than yearly.
A Government spokesman said: "We do not comment on leaks."