Varadkar said Thursday: "We need to know that it's workable and legally operable, and we've yet to see anything that remotely approaches that."
Britain leaves the EU, and its single market and customs arrangement, in March. But the move could create obstacles to business between Britain's Northern Ireland and EU-member Ireland.
EU leaders will assess the Brexit talks' progress next month. They want a deal by October so parliaments can ratify it.
Varadkar said: "If we're not making real, substantial progress by June then we seriously need to question whether we're going to have a withdrawal agreement at all."
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