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The EU has issued its toughest warning yet as talks remain stalled with the UK on the Northern Ireland protocol.
Following talks with Britain's Brexit minister Lord Frost, European Commission vice president Maros Sefcovic threatened the UK with "serious consequences" should it invoke Article 16.
The tough words followed strong words from Lord Frost as he went into talks.
The Northern Ireland Protocol allowed Brexit to happen without a border on the island of Ireland. It provided for customs checks on certain goods crossing the Irish Sea between the island of Ireland and mainland Britain.
Under the terms of the protocol, Article 16 can be invoked by a signatory if it believes it is causing serious economic difficulties. The UK government says it believes that moment came some time ago.
Invoking Article 16 could suspend customs checks across the Irish Sea. That would threaten the European Single Market. It is regarded in Brussels as the nuclear option and could lead to immediate retaliatory consequences.
The EU could suspend the UK-EU trade deal, which could threaten the Brexit agreement itself. The entire post-Brexit settlement could even start to unravel.
Despite that alarming prospect, there was no sign of a breakthrough in these latest talk. Instead, mutual recriminations.
Vice president Sefcovic lamented: "Until today, we have seen no move at all from the UK side. I find this disappointing and once again, I urge the UK government to engage with us sincerely."
For their part, the British complained after the meeting that "progress had been limited and that the EU's proposals did not currently deal effectively with the fundamental difficulties in the way the protocol was operating".
A significant gap remains between the two sides and - although talks will continue - without some skilful creative diplomacy, a further breach in relations seems more than likely, threatening a major rupture in trading relations between the UK and EU.
Mr Sefcovic returns to London for talks next week. Today he called it an important week. We may well be reaching a make or break point in negotiations.