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 British Prime Minister Theresa May on Wednesday.
A parliamentary vote on the deal planned for today 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.
She said she had found a "shared determination" to address MPs' concerns about the proposed backstop arrangement for the Irish border, in order to allow the deal to be brought back to the House of Commons and ratified.
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."