Theresa May's plan to ensure the continuation of a soft border between Northern Ireland and the Republic could be rejected by the European Union, Ireland's foreign minister has suggested.
The Prime Minister has committed to leaving the EU customs union which guarantees tariff-free trade.
However she has insisted a hard border can be avoided through technological solutions and placing no new restrictions on the 80% of cross-frontier trade carried out by smaller businesses.
But the Irish Tanaiste Simon Coveney told BBC One's The Andrew Marr Show he was "not sure that the European Union will be able to support" the plan, as it would be worried about protecting the integrity of the single market.
"While of course we will explore and look at all of the proposed British solutions, they are essentially a starting point in negotiations as opposed to an end point," he said.
Coveney said if agreement cannot be reached during tri-partite talks between the UK, Ireland and the European Commission, the backstop plan of full British alignment with customs union and single market rules that Mrs May "committed clearly" to in December would have to be put in place.
In a major speech on Friday, May rejected "unacceptable" EU proposals to retain customs union arrangements in Northern Ireland, but accepted the UK's "responsibility" to help maintain a soft border with the Republic.
Speaking to Andrew Marr, she declined to defend Boris Johnson's comparison of the border to crossing between London congestion zones in Camden and Islington, but insisted both of them are "absolutely clear" that there will not be a hard border.
"We've got proposals as to how we're going to achieve that, now we're going to be able to sit down and talk with others about how we're going to do that," she said.
May said Britain would align with EU rules in areas like car manufacturing to maintain access to markets but diverge in areas such as fisheries and agriculture.
"Crucially, Parliament will be able to take decisions about the rules that are set and so in the circumstances in which, say, the EU changed a particular rule, there would be a decision for us to take - did we accept it in the future or not?
"But if we didn't accept it, there'd be an arbitration mechanism, an independent arbitration mechanism, so that people would look at it and say actually, you know what, if the UK doesn't accept that, does it make any difference to the trading relationship? And they might say no it doesn't, so there's no consequence, they might say yes it does, and so there would be a consequence," she said.