The DUP has accused Boris Johnson of creating a customs border in everything but name during a House of Commons debate on his Brexit deal.
Mr.Johnson is trying to get MPs to back the deal agreed with the EU earlier in the week.
If every MP casts their vote today, he needs the backing of 320 members of parliament.
During exchanges in the debate, the DUP Deputy Leader, Nigel Dodds, said Northern Ireland must leave with the rest of the UK.
Jeremy Corbyn said his Labour could not support the deal either because it was worse than any of the previous proposals the House of Commons had voted on.
Support for Boris Johnson's plan is therefore on a knife-edge, although some Labour MPs may be willing to go against their party to back the proposals.
It also, however, faces being blocked by an amendment from MP, Oliver Letwin, that would withhold approval of the deal until legislation to enact it is passed.
That would effectively force Boris Johnson to seek a further Brexit extension.
UK Secretary of State, Stephen Barclay, urged Mr.Letwin to reconsider the amendment.
A final vote is not expected to take place until late this afternoon at the earliest.