As was set out in paragraphs 49 and 50 in the Joint report on progress during phase 1 of negotiations under Article 50 TEU on the UK’s orderly withdrawal from the EU:
"The United Kingdom remains committed to protecting North-South cooperation and to its guarantee of avoiding a hard border. Any future arrangements must be compatible with these overarching requirements. The United Kingdom's intention is to achieve these objectives through the overall EU-UK relationship. Should this not be possible, the United Kingdom will propose specific solutions to address the unique circumstances of the island of Ireland. In the absence of agreed solutions, the United Kingdom will maintain full alignment with those rules of the Internal Market and the Customs Union which, now or in the future, support North-South cooperation, the all island economy and the protection of the 1998 Agreement. 50.
In the absence of agreed solutions, as set out in the previous paragraph, the United Kingdom will ensure that no new regulatory barriers develop between Northern Ireland and the rest of the United Kingdom, unless, consistent with the 1998 Agreement, the Northern Ireland Executive and Assembly agree that distinct arrangements are appropriate for Northern Ireland. In all circumstances, the United Kingdom will continue to ensure the same unfettered access for Northern Ireland's businesses to the whole of the United Kingdom internal market.”