The European Union is restarting legal proceedings against the UK as Brussels prepares for a drawn-out fight over London’s move to override part of the Brexit deal.

The UK’s planned legislation announced earlier this week is a breach of international law, Maros Sefcovic, the EU’s chief Brexit negotiator, told reporters on Wednesday. He added that the UK proposal to eliminate the role of the European Court of Justice in governing disputes was a further breach.

“Let there be no doubt: There is no legal, nor political justification whatsoever for unilaterally changing an international agreement,” Sefcovic said. “Opening the door to unilaterally changing an international agreement is a breach of international law as well. Let’s call a spade a spade: This is illegal.”

The simmering years-long dispute between the UK and EU is heating back up, with the bloc restarting legal proceedings against the UK over London’s proposed bill. The legislation would give British ministers the power to unilaterally rewrite the bulk of the Northern Ireland protocol, which keeps the area in the EU’s single market while creating a customs border with the rest of the UK.

UK Prime Minister Boris Johnson is heading for a fresh fight with his own Conservative Party over his proposed bill published Monday, which is likely to run into trouble in Parliament’s unelected upper chamber—the House of Lords—where peers have repeatedly pushed back on efforts to override the Brexit agreement. 

The EU is restarting a case filed against the UK in March 2021 over the implementation of the protocol, and is also launching two new infringement cases over the UK’s failure to carry out health and safety obligations. Infringement proceedings could ultimately lead to financial penalties being imposed on the UK, but the cases will play out over the course of many months.

Asked whether the EU could impose tariffs against the UK, or suspend the bloc’s trade agreement with the UK, Sefcovic replied he expects the bill to “take some time” going through parliament. “Of course if the draft bill will become the law then of course I cannot exclude anything,” he said. “But we are not there yet and we want to solve this issue as two partners should, through negotiations, looking for common ground.”

The European Commission, the EU’s executive arm, on Monday cited the trade deal, saying that “the conclusion of the Withdrawal Agreement was a pre-condition for the negotiation of the Trade and Cooperation Agreement.” The proposed bill “undermines the trust that is necessary for bilateral EU-UK cooperation within the framework of the Trade and Cooperation Agreement,” it said.

Other options open to the EU include stopping the privileged access UK companies have to the single market and halting talks over the status of Gibraltar, according to a person familiar with the matter.

Sefcovic on Wednesday called for more talks with London. “I am still convinced that with genuine political will to make the protocol work, we can reach our objectives,” Sefcovic said. “I call on my UK counterparts to engage in good faith and explore the full potential of the solutions we have put forward.”

EU financial services commissioner Mairead McGuinness told Bloomberg TV earlier Wednesday that Johnson’s push is driven more by internal politics than a concern for the region.

“I think this is about politics in the Conservative Party,” McGuinness said. When the referendum to leave the EU took place six years ago, “do you remember much discussion about Northern Ireland, its needs, its difficulties?”

Johnson’s ministers have shifted the justification for rewriting the protocol, from focusing primarily on trade disruption to the threat to Northern Ireland’s fragile politics. The Democratic Unionist Party balked at the rules Johnson signed up to, and is now refusing to take its place in Northern Ireland’s power-sharing government until the protocol is removed.