Boris Johnson has insisted he is not out to undermine Theresa May by condemning her proposed Brexit deal on a DUP platform.
The Tory Brexiteer said he accepted the invitation to address the Northern Irish party's conference on Saturday in order to "thank them" for helping to keep the UK together.
His intervention in the Brexit debate came on the same day Arlene Foster warned that the DUP would not be "afraid" to reject the prime minister's draft Brexit deal.
The DUP leader told massed crowds in Belfast her country will 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.
Ahead of his speech Mr Johnson reiterated his attack on the backstop, which is the 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.
The former foreign secretary said Britain should threaten to not pay the £39 billion divorce payment in order to force the EU to remove their demand for a backstop as part of the exit agreement.
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.
In her speech, Mrs Foster said: “The Democratic Unionist Party has never been afraid to say yes when it is right to do so, nor to say no when required.
"We do not stand alone on these issues, with a large number of Conservatives - both those who voted leave and remain - not persuaded. Even Jeremy Corbyn isn't buying the sea border.
"This party wants to see a negotiated and orderly withdrawal from the European Union - that has always been our position. We are not campaigning for a no-deal exit nor do we want barriers to trade between Northern Ireland and our neighbours in the Republic.
"The choice is not between this deal and no deal, despite what the Government spin machine may say."
Mrs Foster has said the DUP may revisit its deal to support the Conservatives in key votes at Westminster.
She told the annual conference: “This party acted responsibly in the national interest to secure a government in 2017, and we will not be found wanting in our efforts to secure a better outcome.
"An outcome that does not leave Northern Ireland open to the perils of increased divergence away from the rest of the United Kingdom.
"And an outcome which truly works to the benefit of all parts of this nation."
A government minister from Gibraltar meanwhile told the DUP conference that the British territory “will not fold” in the face of Spain’s Brexit demands.
The position of Gibraltar had been one of the issues facing EU states attempting to agree a common position on UK withdrawal.
Samantha Sacramento was given a rousing reception by the DUP faithful in Belfast as she insisted that Gibraltar will not be “bullied” by any bid from Madrid to undermine its sovereignty.
Housing and equality minister Ms Sacramento was standing in for Gibraltar’s chief minister, Fabian Picardo.
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.
Nigel Dodds, the DUP’s leader at Westminster, told 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," he said.
“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.”