The PM's government partners accused her of "betrayal" and "breaking promises" over the Northern Irish border in a letter she wrote to them earlier this week.
DUP boss Arlene Foster said today that the note, revealed in The Times, "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".
She thinks the letter means the Prime Minister is "wedded" to creating a border down the Irish sea in the event Britain couldn't secure a long-term trade deal with the EU, which would then leave Northern Ireland stuck in the EU's regulatory regime.
But No10 has insisted that Britain won't sign up to anything which divides the UK into "two customs territories".
Mrs May has repeatedly said she will never sign up to anything which splits up the union.
Cabinet Secretary Jeremy Wright insisted that ministers shared concerns about making sure that Northern Ireland was not broken up from the rest of the UK.
He told Radio 4 this morning: "Were there to be a border down the Irish sea, as I have said… we would not accept a deal incorporating that."
The text of the letter says: "I am clear that I could not accept there being any circumstances or conditions in which that ‘backstop to the backstop’, which would break up the UK customs territory, could come in to force.
"That is why it is critical that the provision for a UK-EU joint customs territory is legally binding in the Withdrawal Agreement itself, so that no ‘backstop to a backstop’ is required."
The DUP think that means that the EU's preferred backstop will be written into the legally binding text.
Sammy Wilson said this morning: "From what we can see in the letter sent to Arlene and Nigel, it is quite clear that some of the promises which are made do not conform to some of the contents of the letter which has been written."
The final stages of Brexit talks are underway, with officials on both sides confident a compromise deal can be agreed within days.
A special cabinet meeting next week is expected for Cabinet ministers to look at the final text of a withdrawal agreement.
Mrs May will visit Belgium and France today as part of Armistice celebrations to remember 100 years since the end of World War One.
She will lay a wreath at the graves of the first and last soldiers to die during the horrific conflict.
And she will have lunch with President Macron to discuss Brexit too.
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