Democratic Unionist leader Arlene Foster branded the Irish border backstop "toxic" and said her party's 10 MPs would vote against the Withdrawal Agreement.
Mrs Foster said on Twitter: "Tonight will be historic but for the wrong reasons. We will oppose the toxic backstop & vote against the WA.
"It's time for a sensible deal which governs our exit from the EU & supports all parts of the UK."
The DUP's Westminster leader Nigel Dodds says he still cannot support the withdrawal agreement.
He said: "It doesn't deliver Brexit...in our view and it doesn't safeguard the Union because of the problems with the so-called Irish backstop.
"As things stand we couldn't support Theresa May's deal and I think therefore it will be defeated."