Democratic Unionist Party leader Arlene Foster has said Theresa May's no deal-Brexit plan "raises alarm bells".
The politician said that a letter she had seen from the Prime Minister suggested she was "wedded" to the idea of a customs border in the Irish sea in the case of a no-deal Brexit, leaving Northern Ireland in the European Union's customs union and single market.
In the letter, seen by The Times, Ms May reportedly says she would not accept "any circumstances or conditions" which would "break up the UK customs territory".
'Raises alarm bells'
However, in a post on Twitter, Ms Foster wrote: "The PM’s letter raises alarm bells for those who value the integrity of our precious union and for those who want a proper Brexit for the whole UK.
"From her letter, it appears the PM is wedded to the idea of a border down the Irish Sea with Northern Ireland in the EU single market regulatory regime."
Appearing on the Matthew Wright show, Mick Fealty, the editor of Northern Ireland-based political blog, Slugger O'Toole, said Irish businesses were worried.
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"I was in Dublin a few days ago. There were people saying to us, if we can't sell our cheddar cheese to the English, the French don't want it, the Germans don't want it, how do you actually begin to deal with all of that?," Mr Fealty said.
"The lack of preparation, the lack of thinking through the very complex relationship that we've developed over the last 40 years and how you might untie that without doing huge instrumental damage to say, the dairy industry, which is not only an Ireland-wide industry, it's a UK and Irish Republic-wide industry.
"There is a shared view that this is a disaster."