BELFAST — When it comes to the Irish border, the MPs who prop up Theresa May's government are in no mood to compromise.

Senior figures from Northern Ireland's Democratic Unionist Party, which agreed a confidence and supply deal to keep May's minority government in power after her disastrous June snap election, have advocated a walk-out from Brexit talks if European leaders continue to block progress.

London had previously judged the size of the so-called Brexit bill to be the biggest obstacle to progress to the next phase of divorce talks, with the thorny issue of the Irish border to be negotiated alongside future trade arrangements.

But a leaked update on negotiations circulated earlier this month by the European Commission concluded that the avoidance of “regulatory divergence” on the island of Ireland was “essential” to protect the peace process. Irish Taoiseach Leo Varadkar said this week he was determined to hold firm on his insistence that customs rules should continue to match on either side of the border after Brexit.

European Council President Donald Tusk told May Friday she must find a credible solution for the Irish border if talks are to move onto trade.

Speaking to POLITICO ahead of the DUP's annual conference here, Sammy Wilson, the party's Brexit spokesman and member of the House of Commons Committee on Exiting the European Union, said that a separate arrangement for Northern Ireland was a "red line" the party would not cross.

"They [the Irish Republic] have the most to lose from all of this, so I think they need to stop the posturing and get real" — Jeffrey Donaldson, DUP chief whip

“During the referendum campaign quite a lot of people said Northern Ireland is different, Northern Ireland is the only part of the United Kingdom with a land boundary with a European country so therefore what applies to other parts of the United Kingdom cannot apply to Northern Ireland,” he said. “We were adamant, and this is a red line for us, we will under no circumstances move from this, that we would leave the EU on exactly the same terms as the rest of the United Kingdom did. We would not have some separate arrangement for Northern Ireland just because we have a land boundary."

DUP MPs met Foreign Secretary Boris Johnson for an informal meeting last Tuesday "to talk about some of the Brexit stuff," according to Wilson, who said he and Nigel Dodds, the DUP's leader in Westminster, regularly met ministers to discuss the negotiations.

Since the confidence and supply arrangement was agreed in June, senior figures from the two parties meet at least weekly, sometimes more, according to the party's chief whip Jeffrey Donaldson.

In a sign of the close relationship between the Northern Irish party and their Conservative partners, the Tory party's new chief whip, Julian Smith, will give a keynote speech at the DUP conference Saturday.

Theresa May's de facto deputy, Damian Green, addressed a private dinner on the eve of the event on Friday.

Senior DUP figures, including leader Arlene Foster, also attended the Conservative Party conference in Manchester in October.

Brexit red lines

Donaldson said Friday that a walk-out from Brexit talks could "come at anytime."

"We are not in the business of separating Northern Ireland from the rest of the U.K. in terms of Brexit outcomes," he said. "That means Northern Ireland is leaving the the customs union, Northern Ireland is leaving the single market with the rest of the U.K. and if Dublin want to indulge in unrealistic posturing on this, then I think that harms their own position."

Unionists claim the Irish Republic would be "the biggest loser" in a no-deal Brexit scenario given its strong trade ties with the U.K.

"They have the most to lose from all of this, so I think they need to stop the posturing and get real about the fact Northern Ireland is leaving the single market and the customs union and therefore it is in their interest to get a customs agreement and a trade agreement with the EU," the DUP chief whip added.

Dublin's tough talk on the Irish border question in recent weeks has overshadowed May's previous cabinet stalemate over the scale of financial obligations the U.K. would be willing to pay.

We have a big trade deficit with the EU and we shouldn’t underestimate the leverage we have in this process" — Jeffrey Donaldson

May reportedly received the backing of her Cabinet, including prominent Brexiteers Boris Johnson and Michael Gove, to increase her offer to EU leaders ahead of the December EU summit.

But while placating Johnson and Gove (for now) she could yet come under pressure from senior DUP figures, who advocate a harder line on the question of cash.

Donaldson said May should "hold the line" on the financial settlement and "not be bullied into accepting a high price."

"In the end, the U.K. is a very valuable market for the EU, we have a big trade deficit with the EU and we shouldn’t underestimate the leverage we have in this process," he added.

Wilson suggested the U.K. government had allowed themselves to be "spooked a wee bit" and said he had been "disappointed" the way it had "made a mess of our contributions."

He said he was "surprised" Johnson and Gove had "acceded to giving more money," though he acknowledged they had a better insight into the negotiations and party pressures.