The backstop must be removed from the Withdrawal Agreement, Arlene Foster has insisted.
The Democratic Unionist leader was speaking ahead of a planned meeting with Prime Minister Theresa May on Wednesday.
A parliamentary vote on the deal planned for Tuesday was called off as it became clear it was unlikely it would be passed by MPs.
Mrs May spent the day instead in talks with fellow leaders, including Germany’s Chancellor Angela Merkel and Mark Rutte, prime minister of the Netherlands.
Mrs Foster said she has a simple message for Mrs May – that the backstop must be removed from the deal.
“This has been our message from the day a backstop was conceived,” she said.
“With London, Dublin and Brussels guaranteeing that no-one will build a hard border between NI and RoI in any circumstances, there is no genuine need for the backstop.
“The electorate voted to leave the EU and that vote must be respected.
“People wanted Parliament to take back control of our money, laws and borders.
“The Prime Minister’s flawed Withdrawal Agreement gifted control back to Brussels.
“That is why Parliament would have been right to reject it.”
Mrs Foster emphasised she was seeking more than assurances.
“We want to see the Withdrawal Agreement fundamentally changed,” she said.
“The Prime Minister must stand up to the EU and robustly defend the constitutional and economic integrity of the United Kingdom.
“I am very grateful for the support we have received from across the House in opposing this dangerous Withdrawal Agreement.”
Meanwhile Sinn Fein President Mary-Lou McDonald said she told Mrs May in a phone call on Tuesday evening that the backstop is “non-negotiable”.
She said that along with Irish TD Pearse Doherty, she spoke to Mrs May for 20 minutes, during which time she raised concerns about her “course of action”.
“We told the British Prime Minister that the basic protections contained in the backstop are non-negotiable and cannot be unpicked or diluted,” she said.
“We raised concerns that we are facing into a no deal or a crash Brexit which would be a disaster for Ireland.
“And we reminded Mrs May that, in those circumstances, a Unity Referendum must be called as a matter of urgency.
“As I told the Taoiseach today, Irish Unity is the ultimate contingency to protect our interests in the event of a crash Brexit.”