Arlene Foster warns DUP will revisit support for UK government if May's Brexit deal passed
The draft EU withdrawal agreement contains arrangements that are not in Northern Ireland's long-term interests, Arlene Foster will tell her party conference later.
The country would inevitably begin to diverge from its largest market, the rest of the UK, if it remained part of the EU's customs code under the backstop arrangement, the DUP leader will declare before massed crowds of supporters in Belfast.
The backstop is a fallback designed to avoid a hard Irish border which would see Northern Ireland continue to follow some EU rules around trade if no better solution can be found.
Mrs Foster is expected to say: "We cannot wish away the fact that the draft withdrawal agreement contains arrangements that are not in Northern Ireland's long-term economic or strategic interests.
"Northern Ireland would remain part of the European Union's customs code and as things stand we would be sowing the seeds of inevitable economic divergence from our largest market."
Tory Brexiteer Boris Johnson is also expected to address the DUP conference on Saturday.
Mrs Foster acknowledged the frustration of many in Northern Ireland's business community with the pace of negotiations.
Her party has prided itself on its relationship with firms, but relations have been strained by their support for the draft UK-EU deal.
Mrs Foster has said the DUP may revisit its deal to support the Conservatives in key votes at Westminster.
She will tell her annual conference: "I acknowledge the hard work and determined efforts of the Prime Minister to secure an agreement.
"She is genuine when she says she wants to see an outcome that does no harm to the Union and the internal market of the United Kingdom.
"However, this draft agreement fails her own key commitments.
"The Prime Minister has not been able to guarantee an outcome that eliminates the risk of the introduction of the so-called backstop arrangements."
Mrs Foster is expected to urge negotiators to seek a better outcome.
The position of Gibraltar had been one of the issues facing EU states attempting to agree a common position on UK withdrawal.
Spain is pressing for a guarantee over its say on the future of Gibraltar.
A delegation from the British overseas territory on the southern tip of the Iberian peninsula travelled to take part in the DUP conference.
The DUP leader is expected to pledge: "We stand with the people and the government of Gibraltar at this time."
Nigel Dodds, the DUP's leader at Westminster, is expected to tell the conference that without his party's support the Government would have been defeated in almost half of all votes on the EU Withdrawal Act.
"The Government's commitments under it are clear... including on Brexit. Commitments freely entered into must be delivered and if they are not, then clearly, as we have shown, there are consequences.
"But it is in the interests of the country and of Northern Ireland that our parties continue to work together in the national interest.
"Our agreement is of course with the Conservative Party.
"It is incumbent therefore on all Conservatives in Parliament who recognise the importance of continuing stability and who wish to see the Government deliver its agenda to ensure it is honoured in full."