Theresa May has told the Tory party conference that she will never accept a Brexit deal that "carves off" Northern Ireland.
The embattled Conservative leader said nobody wanted a good withdrawal agreement more than she did but that she would not accept a "deal at any cost".
"Britain isn't afraid to leave with no deal if we have to but we need to be honest about it," she said.
"Leaving without a deal – introducing tariffs and costly checks at the border – would be a bad outcome for the UK and the EU."
Mrs May said a no deal scenario would be "tough at first" but that Britain would pull through thanks to the "resilience and ingenuity" of its people.
She said ruling out a no deal would weaken the UK's negotiating position, leaving her to accept a deal that keeps Britain "in the EU in all but name" or one that "carves off Northern Ireland" and leaves the region in the Custom's Union.
The prime minister said she would "never accept either of those choices".
"We will not betray the result of the referendum and we will never break up our country," she told the conference in Birmingham.
She said her government's proposal was for a "free trade deal that provides for frictionless trade in goods".
It would protect hundreds of thousands of jobs, she said, while protecting "our precious union".
"The seamless border in Northern Ireland, a bedrock of peace and stability, would see no change whatsoever," she said.
"No simple free trade agreement could achieve that, not even one that makes use of the very latest technology."
The Tory leader repeated her call for "respect" from the European Union and stressed that she would not accept "a deal at any cost".
Mrs May said Britain's greatest strength was the "talent and diversity of our people".
"Don't let anyone tell you we don't have what it takes: we have everything we need to succeed," she said.
"And in 2022 we will put the best of British creativity and innovation, culture and heritage on show in a year-long festival of Great Britain and Northern Ireland."
