Britain is set to be handed a final warning from the European Union to meet its commitments under the Northern Ireland Protocol as the two sides struggle to work out their post-Brexit relationship.

The European Commission, the EU’s executive arm, plans to file a so-called reasoned opinion on what it says are the U.K.’s breaches of the protocol by the end of the month, according to two officials with knowledge of the process.

If the U.K. government refuses to back down, the commission could then refer the case to the EU’s Court of Justice in Luxembourg. A spokesperson for the commission declined to comment.

The dispute is escalating after David Frost, the British minister for EU affairs, on Wednesday demanded that the bloc renegotiate the protocol, saying that it has created an unsustainable situation with “significant disruption” to trade between the Northern Ireland and the rest of the U.K. Frost, who helped to draft the protocol as Britain’s chief Brexit negotiator, said it is creating instability in the province.

The EU rejected his appeal out of hand.

“We will not agree to a renegotiation,” European Commission Vice President Maros Sefcovic said in a statement on Wednesday. “Respecting international legal obligations is of paramount importance.”

Under the terms of the deal on Northern Ireland, some goods shipped there from Britain face customs checks and procedures as if they are crossing into the EU.

British Prime Minister Boris Johnson opted for those arrangements so that the rest of the U.K. would have the freedom to diverge from EU market rules after leaving the bloc. Both sides agreed early on in the Brexit process that the alternative, border checks between Northern Ireland and the Irish Republic, would risk jeopardizing the 1998 peace settlement that ended decades of sectarian violence.

Irish Sea Border

Johnson’s predecessor Theresa May had argued that creating such a separation between two parts of the U.K. was something that no British leader could countenance. Politicians in Northern Ireland and London have since complained that the effective trade border in the Irish Sea threatens the territorial integrity of the U.K. and has led to a reduction in trade flows to the region.

The EU has long insisted that the protocol must be implemented as originally designed, in order to protect its single market and prevent Northern Ireland being used as a back door for smuggling into the 27-nation bloc. In March, the EU began legal action against the U.K. after Britain unilaterally delayed enforcing part of the agreement.

The EU has since agreed to a grace period that allows British suppliers to continue shipping sausages and chilled meats into Northern Ireland until the end of September. After, such trade could be banned because the EU doesn’t allow such products into its single market for health and safety reasons.

If there is no resolution, Frost raised the possibility of triggering Article 16 of the protocol, which allows either side to suspend elements of the agreement if it is causing serious problems. The EU would then have to right to take measures of its own in response.