Prime Minister Theresa May has repeated her pledge that she will never accept a Brexit deal which carves Northern Ireland away from the rest of the UK.
She said current EU proposals would effectively leave NI in the Custom's Union, unlike other parts of the UK.
She also insisted her Chequers plan will deliver the best outcome for the UK post Brexit and called on her party to unite behind it.
Mrs May was addressing the Conservative Party conference in Birmingham.
In a speech full of optimism, the PM insisted that the UK's best days are still to come after Brexit.
But she insisted the option of leaving the EU without an overall deal must be kept on the table.
To remove that option, Mrs May claimed, would mean accepting one of two things:
"Either a deal that keeps us in the EU in all but name, keeps free movement, keeps vast annual payments and stops us signing trade deals with other countries," she said.
"Or a deal that carves off Northern Ireland, a part of this country, effectively leaving it in the EU's Custom's Union."
The prime minister urged the conference to send a clear message that the Conservative Party will "never accept either of those choices".
"We will not betray the result of the referendum and we will never break up our country," Mrs May said.
She added that her government was now entering the toughest phase of the negotiations and she urged her party to rally behind her under her much criticised Chequers plan.