The UK will have "no choice but to act" unless the European Union agrees to change post-Brexit trading arrangements in Northern Ireland, Foreign Secretary Liz Truss said on Thursday.
Political tensions have risen in the UK province after elections last week saw pro-Irish nationalists Sinn Fein become the biggest party for the first time and now bid to lead a power-sharing executive.
But pro-UK unionists, who believe the Brexit deal's Northern Ireland Protocol is driving a wedge between the province and mainland Great Britain, are refusing to join the executive in Belfast until it is changed.
Truss spoke to European Commission vice president Maros Sefcovic on Thursday, telling him that the situation was "a matter of internal peace and security for the United Kingdom.
If the EU does not "show the requisite flexibility to help solve those issues, then as a responsible government we would have no choice but to act," she said.
London has repeatedly said it is prepared to trigger the protocol's Article 16 suspension clause unless the deal it signed up to is changed -- a move the EU has warned could lead to a wider trade war.
Sefcovic responded by warning that any move by the UK ignore post-Brexit trade rules in Northern Ireland would be unacceptable and threaten the 1998 peace deal that ended decades of violence over British rule in the province.
"Unilateral action... is simply not acceptable," Sefcovic said in a statement.
"This would undermine trust between the EU and UK as well as compromise our ultimate objective – to protect the Good Friday (Belfast) Agreement.
"Upholding the rule of law and living up to international obligations is a necessity," he added.
- 'Two-tier system' -
Either party can invoke Article 16 if it believes the protocol is leading to "serious economic, societal or environmental difficulties that are liable to persist".
The UK government says that checks on goods heading from to Northern Ireland from England, Scotland and Wales are undermining peace in the province, with unionists protests already turning violent.
Separate trading arrangements for Northern Ireland, which bind the province bound to many European rules, were agreed because the province has the UK's only land border with the EU.
Keeping the border open with neighbouring Ireland, an EU member, was mandated in the Good Friday Agreement, given the frontier was a frequent flashpoint for violence.
But it means checks have to be done elsewhere, to prevent goods getting into the EU single market and customs union by the back door via Northern Ireland.
Unionists believe it has created a border in the Irish Sea which threatens Northern Ireland's place as part of the wider UK, and makes a united Ireland more likely.
Truss told Sefcovic that the UK's priority was "to protect peace and stability in Northern Ireland".
"She also noted that the current situation was causing unacceptable disruption to trade and had created a two-tier system where people in Northern Ireland weren't being treated the same as everyone else in the UK," according to a read out of the phone call released by her office.
Truss added that the EU "bore a responsibility to show more pragmatism and ensure the Protocol delivered on its original objectives," claiming that the government had proposed measures to remove trade barriers within the UK while protecting the EU single market.
Sefcovic said that the EU had already proposed "wide-ranging and impactful solutions" that would "substantially improve the way the protocol is implemented," but that London had rejected them.