In a statement, the Democratic Unionist Party, which the government relies on for support in key votes, said: "These proposals are not, in our view, beneficial to the economic well-being of Northern Ireland and they undermine the integrity of the Union."
It added: "Saturday's vote in Parliament on the proposals will only be the start of a long process to get any Withdrawal Agreement Bill through the House of Commons."
A Brexit deal has been agreed between the UK and EU before a meeting of European leaders in Brussels.
Boris Johnson and Jean-Claude Juncker called it a "fair" outcome - and the EU Commission President said there was no need to extend the Brexit deadline.
He said: "We have a deal so why should we have a prolongation."
This will be a boost for the PM, but he still faces a battle to get the deal through Parliament on Saturday, with the DUP opposing it.
Mr Johnson urged MPs to "come together" and "get this excellent deal over the line".
He added: "Now is the moment for us to get Brexit done and then together to work on building our future partnership, which I think can be incredibly positive both for the UK and for the EU."