The party leader, Arlene Foster, accused Theresa May in her Friday tweet of doing exactly what the PM vowed not to do — allow a hard border between Northern Ireland and the Republic of Ireland post-Brexit.
The social media post comes as the Democratic Unionist Party (DUP) party responded to May's letter addressed to Foster, which was leaked to the Times.
Foster claimed that the PM's letter raised "alarm bells for those who value the integrity" of the United Kingdom.
In the letter, Theresa May said she would not allow an Irish border backstop that separated Northern Ireland from the rest of the UK to ever come into force.
For the DUP however this meant that Westminster will agree to a Brexit deal that includes a backstop arrangement, something the party has opposed from the start.
"From her letter, it appears the PM is wedded to the idea of a border down the Irish Sea with NI in the EU SM regulatory regime," Foster said in her post.
The PM's letter raises alarm bells for those who value the integrity of our precious union & 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 NI in the EU SM regulatory regime. https://t.co/qN6c41zx7U— Arlene Foster (@DUPleader) November 9, 2018
The DUP are propping May's government in Westminster following the 2017 general election, when the Conservatives forged a coalition with the Northern Irish unionists to retain the majority in the parliament.
But when it comes to Brexit, the DUP were determined to oppose any deal suggesting a divide between Belfast and the rest of Britain was a "red line" for the unionists.
On Friday, the DUP called on the UK government to provide clarity on the ongoing Brexit negotiations.
"Unlike the previous political declarations, the scope for delay, fudge or obscurantist language has passed. This is now a time for clarity and plain speaking," the DUP said in a Twitter post featuring the party letter to Theresa May dates November 1.
2/2 1st November 2018 letter from Arlene Foster @DUPleader & Nigel @NigelDoddsDUP to the Prime Minister. "It is vital for both NI & the entire UK that if agreement is reached, it must be clear and must not risk opening divergence within the UK for generations to come." pic.twitter.com/gCcvp01p8O— DUP (@duponline) November 9, 2018
The spokesman for the DUP, Sammy Wilson, confirmed that regulations cooked in Europe and then imposed on Belfast would result in Northern Ireland being cut off from the UK.
Earlier this week, Irish Prime Minister Leo Varadkar, expressed doubt a special November summit would see a deal framework complete. He suggested the December EU summit was a more likely scenario to get the agreement ready.
"…we do have one scheduled for the 13th, 14th of December, so not getting it done in November doesn't mean we can't get it done in the first two weeks of December. But I think beyond that you're into the New Year, which I think wouldn't be a good thing," the Irish PM warned.
The Irish government is willing to consider a review clause attached to the backstop and agreed with Theresa May that such solution should be temporary.
A "backstop" arrangement was proposed as part of the Britain's withdrawal deal from the EU. It would ensure an open border by keeping Northern Ireland inside the EU customs union and large parts of the single market. Britain has suggested a UK-wide backstop, with Britain remaining aligned with the EU customs union for a limited time after 2020.