Liz Truss will insist on Sunday that Britain will overhaul the post-Brexit agreement over Northern Ireland unilaterally if she cannot reach a "negotiated solution" with the EU.
Writing in The Telegraph in her first intervention as Boris Johnson's new Brexit negotiator, the Foreign Secretary states that the Article 16 "safeguard clause" in the Northern Ireland Protocol was "explicitly designed ... to ease acute problems because of the sensitivity of the issues at play" on the island of Ireland.
The warning, which is likely to inflame tensions with Brussels, comes ahead of Ms Truss's first face-to-face meeting with Maros Sefcovic, her EU counterpart, at Chevening, the Foreign Secretary's country residence, on Thursday.
In early November, Mr Sefcovic warned of "serious consequences" if the UK triggered Article 16, which allows either side to take unilateral "safeguard measures" if the protocol causes "serious economic, societal or environmental difficulties".
Until now, senior EU officials had believed that the Government had "shelved" a back-up plan to trigger Article 16, partly as a result of a series of political rows which were seen as having severely weakened the Prime Minister's political standing. Both sides also ideally want a resolution before Northern Ireland's election campaigns begin in March.
Lord Frost, Ms Truss's predecessor as the Government's post-Brexit negotiator, was seen as one of the strongest advocates of deploying Article 16 to unilaterally lift measures such as checks on goods bound for Northern Ireland from Great Britain.
However, drawing on the EU's aborted move to trigger the clause last year over its Covid vaccine rollout, Ms Truss states: "My priority is to protect peace and stability in Northern Ireland. I want a negotiated solution but if we have to use legitimate provisions including Article 16, I am willing to do that.
"This safeguard clause was explicitly designed - and agreed to by all sides - to ease acute problems because of the sensitivity of the issues at play.
"The EU has already invoked this article to introduce a hard border for vaccine exports and, even in the act of withdrawing it, insisted on its right to do so again in the future."
Separately, a senior minister insisted that Mr Johnson must take a hard line on the potential use of Article 16 to lift measures imposed by the protocol, which the Government insists has had a chilling effect on trade in Northern Ireland. While some Cabinet ministers were "weak-kneed" over the potential for a major row with the EU, "the PM is at the hard end of this", the minister said.
Last month, The Telegraph disclosed that Rishi Sunak, the Chancellor, had argued against collapsing the talks before the new year.
Truss sticks to Frost's hard lines
Ms Truss, who along with Mr Sunak is seen as a frontrunner to succeed Mr Johnson, has faced claims that she would take a softer approach to the negotiations than her predecessor, having campaigned for Remain during the 2016 Brexit referendum.
But her article suggests that she will maintain many of Lord Frost's specific demands, including that there must be "no checks or documentation for goods moving from Great Britain to Northern Ireland and staying there".
Ms Truss adds: "I am prepared to work night and day to negotiate a solution. But let me be clear, I will not sign up to anything which sees the people of Northern Ireland unable to benefit from the same decisions on taxation and spending as the rest of the UK, or which still sees goods moving within our own country being subject to checks."
The Foreign Secretary also insists that "we need to end the role of the European Court of Justice as the final arbiter of disputes."
João Vale de Almeida, the EU's ambassador to the UK, who will take part in the talks on Thursday, said: "The EU is focused on finding practical solutions for the problems Brexit created in Northern Ireland and which the protocol aims to mitigate. Invoking Article 16 or raising issues which are not the core concern of citizens and businesses in Northern Ireland will not contribute to solving problems. It will only create more problems by prolonging uncertainty and risking further instability in Northern Ireland."
The ambassador suggested that he hoped Ms Truss would "reciprocate" proposals issued by the EU in October, with "practical solutions for outstanding issues within the obligations we jointly committed to respect".
Meanwhile, Anne-Marie Trevelyan, the International Trade Secretary, will visit India this week to launch trade talks with the country.
As part of a two-day visit to New Delhi, the Cabinet minister is expected to hold talks about "the removal of market access barriers for both UK and Indian businesses" and to formally launch negotiations over a post-Brexit free trade deal.
Ms Trevelyan said: "I will be using my visit to drive forward an ambitious trade agenda which represents the UK's Indo-Pacific tilt in action and shows how we are seizing global opportunities as an independent trading nation."