It is not possible to see how the Irish border issue can be resolved after Brexit, the influential group of MPs scrutinising the process has said.
The government wants no hard border between the Irish Republic and Northern Ireland and no customs border between the latter and the rest of the UK.
Ministers have suggested technology could enable a "frictionless border".
But the Committee for Exiting the EU said the proposals were "untested" and "to some extent speculative".
Northern Ireland will be the only part of the UK to share a land border with an EU member state after the UK leaves.
There is currently no physical infrastructure on the border but there is concern that this will have to change after Brexit.
If the UK leaves the EU's single market and customs union, as the government intends, the Irish land border will become the external border for the EU's single market and customs union.
The Irish Republic wants Northern Ireland to keep following EU rules, so that goods can continue moving across the border - in effect, staying within the customs union and single market.
But this would effectively push the customs border out into the Irish Sea, becoming an internal customs border between Northern Ireland and the rest of the UK - which the UK government rejects.
In its report, the Exiting the European Union Committee says it does not see how it will be possible to reconcile these positions
The government has put forward two proposals, one using "technology-based solutions", such as pre-screening of goods and trusted trader schemes, to reduce the need for customs checks at the Irish border.
The other would involve a "customs partnership", with the UK leaving the single market without introducing an EU-UK border - something the UK has admitted would be "challenging".
The committee is urging the government to set these proposals out in more detail.
http://www.bbc.co.uk/news/uk-politics-42188485
