BRUSSELS (AP) — The European Union is warning Britain that any breach, even the most minor detail, of the withdrawal treaty both sides signed, would break international law and undermine the little trust that is left between both sides negotiating a future post-Brexit trade agreement.

European Commission President Ursula von der Leyen insisted the age-old diplomatic cornerstone of “agreements must be kept" continues to apply and called the principle “the foundation of prosperous future relations."

She said if the U.K. follows through on indications that it's prepared to break an international agreement on EU-British relations, it “would break international law and undermines trust."

On Tuesday, Northern Ireland Secretary Brandon Lewis appeared to admit as much when he told lawmakers that legislation to be published Wednesday would change aspects of the Brexit withdrawal agreement between the U.K. and the EU.