The European Union has launched fresh legal action against the UK for failing to comply with the Northern Ireland Protocol.
It instigated four new infringement procedures on Friday as relations between the bloc and Westminster further soured.
The EU said it was forced to act as the UK has shown an "unwillingness to engage in meaningful discussion" since last February on the post-Brexit deal aimed at avoiding a hard border on the island of Ireland.
Its executive said it had refrained from launching legal action for more than a year "to create the space to look for joint solutions with the UK".
But it said the fact the Northern Ireland Protocol Bill continues to go through the UK parliament goes "directly against this spirit" of cooperation.
The bill is aimed at ripping up parts of the protocol, including removing checks on goods and animal products going from Great Britain to Northern Ireland, which the government has said is necessary to safeguard peace and stability provided by the Good Friday Agreement.
On Wednesday, the bill - which the EU says is illegal - cleared the House of Commons to give it a third reading by the House of Lords after the summer break.
The European Commission accused the UK of failing to comply with customs and excise requirements and not imposing EU rules on VAT for e-commerce, as well as failing to collect relevant export declaration data on goods moving from Northern Ireland to the rest of the UK.
It said failing to do so is increasing the risk of smuggling via Northern Ireland.
An EU document revealing details of the legal action said: "For example, it opens the possibility for traders to circumvent EU rules on prohibitions and restrictions on the export of goods to third countries or provides possibilities for carousel trafficking of goods being declared for export in the EU and actually not exiting the customs territory via Northern Ireland."
It added that the UK has two months to reply to the EU "after which the Commission stands ready to take further measures".
Former lead Brexit negotiator Lord Frost said the move by the EU is political.
"Those who still think that the CJEU's (Court of Justice of the European Union) role in Northern Ireland is just a theoretical or ideological issue may want to think again," he said.
And Tory leadership contestant, and former chancellor, Rishi Sunak, accused the EU of "interfering with our sovereign right to control our own e-commerce rules and set our own taxes".
A spokesman said Mr Sunak would continue with the Northern Ireland Protocol bill unless the EU is willing to come to the table to renegotiate the protocol.
He said today's actions show the EU is "committed to taking petty and unwelcome steps that infringe on our sovereign rights, rather than addressing the real issues around the protocol".
This is not the first time the EU has launched legal action over the protocol.
In June, the EU launched two legal infringement actions against the UK over the protocol.
At the same time, Brussels resumed proceedings against the UK it had suspended last September for breaching the EU withdrawal treaty agreed in 2020.
It threatened to take the UK to the European Court of Justice if there was no reply in two months.
The Northern Ireland Protocol has effectively created a border in the Irish Sea between Great Britain and Northern Ireland, meaning goods exported from Britain are subject to customs checks.
The UK government says that has created a headache for businesses and the power-sharing arrangements set up as a result of the Good Friday Agreement in 1998.
But the EU says it is "the best solution" the bloc and the best resolution the UK could come up with to protect the agreement, avoid a hard border and ensure integrity of the EU's single market.