The British government on Tuesday condemned threats to port workers carrying out post-Brexit trade checks in Northern Ireland.
Checks were suspended at Belfast and Larne ports late on Monday, and the European Commission said staff working at two border posts in the region were told to stay away.
The development comes a little more than a month into new trading arrangements after Brexit took full effect, amid warnings it could stoke lingering sectarian sentiment in Northern Ireland.
UK minister Michael Gove called the situation "completely unacceptable", echoing condemnation from the power-sharing Northern Ireland Executive and the European Commission.
"It's vital that everyone in Northern Ireland, and indeed the UK, exercises calmness and moderation as well as resolution to resolve the problems," he told Parliament.
Regulatory animal-based food checks were stopped on the orders of Northern Ireland's Department of Agriculture, Environment and Rural Affairs.
At Larne Port, 12 staff were pulled from duties after "an upsurge in sinister and menacing behaviour in recent weeks", the Mid and East Antrim Borough Council said.
Pro-British loyalists have been blamed for graffiti criticising the Northern Ireland Protocol and describing port staff as "targets".
Attempts also were reported to have been made to collect information on employees, including vehicle registration plates, in acts of intimidation recalling the darkest days of the Troubles in Northern Ireland.
Police patrols have since been increased, while Northern Ireland's leaders are scheduled to talk to UK and EU ministers on Wednesday.
The Northern Ireland Protocol came into effect on January 1, when the Brexit transition period ended and the full effects of the UK's 2016 decision to split from the EU were eventually felt.
The protocol is designed to prevent a hard border emerging between the north and EU member the Republic of Ireland – a frequent flashpoint in three decades of violence over British rule.
Up to 1998, more than 3,500 people were killed as unionists, who back ties to Britain, engaged in a deadly tug-of-war with republicans seeking to merge the province with the Republic of Ireland.
Security checkpoints and patrols along the 500-kilometre border were attacked by republican paramilitaries in some of the bloodiest chapters of the sectarian violence.
Post-Brexit, London and Brussels feared splinter republican groups still active after the 1998 Good Friday Agreement peace deal could target checks and use them as a recruiting tool to grow their base.
The protocol avoids the need for a hard border by transplanting EU single market and customs union checks to Northern Irish ports and airports.
However, some unionists believe these create a de facto sea border between Northern Ireland and Great Britain – England, Scotland and Wales.
They feel the border disrupts the principle of free movement between the united territories and binds Northern Ireland in an economic union with the Republic.
Unionists – and more extreme loyalists sometimes linked to paramilitaries – called 2019 legislation to enact the Northern Ireland protocol "Betrayal Act".
Last week, the Police Service of Northern Ireland said officers were noticing "growing discontent" among the unionist community – chiefly graffiti and on social media.
The protocol became the renewed focus of unionist ire last weekend after a coronavirus vaccine row pushed the European Commission to warn it would invoke Article 16 of the treaty to restrict vaccine exports to Northern Ireland.
The fallback provision allows London or Brussels to unilaterally suspend aspects of the deal in special circumstances.
Brussels quickly backtracked on the threat – now widely considered a diplomatic bungle – after criticism from the UK, the Republic of Ireland and all of Northern Ireland's mainstream political parties.
Mr Gove said the EU decision on Friday was a "serious mistake" and that "trust has been eroded" between London and Brussels.
"The EU needs to work with us at speed and with determination to resolve the serious outstanding issues with the protocol," he told MPs.
