European Union governments agreed on the need for "robust" action against Britain if London follows through on its threat to escalate a post-Brexit trade dispute, EU diplomats said on Wednesday.
Britain and the European Commission have been in talks for the past month over trading arrangements for Northern Ireland, with checks required for goods entering from Britain.
The checks are needed to avoid introducing a hard border between the British province and EU member Ireland.
European Commission Vice-President Maros Sefcovic briefed EU ambassadors in Brussels on Wednesday and, according to EU diplomats, told them that engaging with Britain was not going well, but that talks should continue.
The Commission last month offered Britain a package of measures to ease transit of goods to Northern Ireland, but it stopped short of the overhaul London wants of the protocol that governs the province's unique trading position.
Britain has threatened to trigger emergency provisions under Article 16 of the protocol.
"The EU is preparing for the triggering of Article 16 by the UK. There is consensus among EU member states that such an arbitrary and unjustified move by the UK will be met with a clear European response," one EU diplomat said.
EU diplomats said there was agreement across the 27 EU members that such a clear response would be needed.
Some stressed it should be "proportionate", others that it should be "hard-hitting" or "robust", even if this also had a negative impact on the EU economy.
"There is no point just being strung along," one diplomat said.
The Commission, which oversees EU relations with post-Brexit Britain, may present possible options for action next week. EU leaders could also convene to determine the response if Britain did indeed trigger Article 16.
British Brexit Minister David Frost said on Wednesday that Brussels should stay calm and avoid embarking on "massive and disproportionate" retaliation.
