If the House of Commons rejects the new Brexit deal, Prime Minister Boris Johnson will be forced by law to ask the EU for a delay. But several EU leaders, including French President Emmanuel Macron, are growing impatient with London.
If MPs vote against Johnson's deal today, it will trigger the third delay to Brexit in as many years.
French President Emmanuel Macron has piled pressure on MPs by saying he did not want a new delay now a deal was struck.
"The 31 October date should be respected. I don't think that new deadlines should be given," he said at the EU summit in Brussels on Thursday.
"We need to end these negotiations and get on negotiating the future relationship."
Luxembourg's Prime Minister Xavier Bettel added: "There is no choice between Brexit or no Brexit: it's a choice between deal or no deal."
Rejected three times
Johnson took office in July vowing to keep to the October 31 Brexit deadline, deal or no deal.
He pledged to renegotiate the most contentious elements of a divorce text agreed by his predecessor Theresa May with Brussels last year, which MPs rejected three times.
European Commission President Jean-Claude Juncker warned such a rejection would create an "extremely complicated situation."
"We have a deal, and this deal means there is no need for any kind of prolongation," he told reporters, although the decision will be for EU leaders.
The draft agreement was forged after weeks of tense negotiations focused on altering arrangements to keep open the border between British Northern Ireland and EU member Ireland.
All sides agree they do not want infrastructure on the frontier, to avoid exacerbating tensions over Britain's control of Northern Ireland that caused decades of deadly violence up until the 1990s.
The compromise deal that was finally struck on Thursday has a new arrangement for keeping open the border between British Northern Ireland and EU member Ireland.
The DUP has said it cannot support the plans, as efforts to avoid checks on the Irish land border would lead to new trade barriers between Northern Ireland and mainland Britain.