EU challenges a British government bid to scrap parts of post-Brexit trade arrangements in Northern Ireland.
The European Commission has launched four new legal proceedings against Britain over London's failure to implement Brexit divorce terms to govern trade with Northern Ireland.
"Despite repeated calls by the European Parliament, the 27 EU member states and the European Commission to implement the Protocol (on Northern Ireland), the UK government has failed to do so," a statement said on Friday.
The proceedings by Brussels add to the legal caseload after a year-long truce as the EU challenges a British government bid to scrap parts of post-Brexit trade arrangements in Northern Ireland.
The UK government last month unveiled legislation to unilaterally change trading terms for the politically-fraught British province and the plan is now making its way through parliament.
"In the spirit of constructive cooperation, we have refrained from launching new internal procedures for over a year to create the space to look for joint solutions with the UK," said EU spokeswoman Arianna Podesta.
"However, the UK's unwillingness to engage with them in a meaningful manner in discussions since last February, and the continued passage of the Northern Ireland Bill through the UK Parliament go directly against the spirit of cooperation."
The EU also accused the UK of investing "little time in explaining the Protocol to people and businesses in Northern Ireland, and did not prepare them adequately for the changes Brexit caused".
The EU said the new proceedings target a series of violations including failure to implement the EU's custom rules, which was raising the chances for illegal products entering Europe and facilitating fraud.
Britain was also accused of failing to implement EU alcohol excise taxes.
The four legal proceedings will be added to existing ones that also target trade matters on the island of Ireland.
In one of those cases, involving food imports, the EU on Friday warned that it would take Britain to the blocs European Court of Justice if London failed to take the measures required.
Under the terms of its divorce, Brussels could also suspend portions of the EU-UK trade agreement, sparking a trade war when the economy in Europe is already fragile over a real war in Ukraine.
The protocol requires checks on goods arriving into Northern Ireland from England, Scotland and Wales, to track products that could be potentially headed onwards to the bloc's single market via the Republic of Ireland.
This creates a customs border down the Irish Sea, keeping Northern Ireland in the EU's customs orbit to avoid a politically-sensitive hard border between the British territory and EU member Ireland.