A report from the British House of Commons Committee on Brexit says it can't see how a hard border can be avoided.
The Committee says the British government's plan to leave the Customs Union and the Single market can't be reconciled with there being no border.
It comes as the Taoiseach prepares to meet the EU Council president Donald Tusk in Dublin.
They'll be discussing progress in the Brexit talks, ahead of this month's crucial EU summit.
They've both floated the idea of the north staying in the Customs Union and the Single Market, in order to prevent a hard border.
But DUP deputy leader Nigel Dodds says that's not acceptable to his party:
