Democratic Unionist Party (DUP) leader Arlene Foster has refused to rule out collapsing the British government if her “blood red line” on Brexit – not to place a customs border between Northern Ireland and the rest of the U.K. – isn’t met.

Prior to visiting Taoiseach Leo Varadkar in Dublin on Monday, Foster called for “calm heads” ahead of the midweek EU summit, but said she was “very clear” in her demands.

As fear mounted that a no-deal Brexit was more likely than ever, Foster told reporters, “We need to see that the whole of the United Kingdom leaves the EU together and there aren’t differences made between Northern Ireland and any other parts of the United Kingdom.”

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The DUP leader also met Fianna Fail leader Micheal Martin during her Dublin visit.

Foster said, “Great Britain is our largest market by far and we cannot have barriers.”

Following her dinner meeting with Varadkar, a spokesman for the taoiseach said they had a good discussion in “a very pleasant atmosphere.”

But the Irish Examiner reported that Foster is also understood to have privately told Varadkar that she will not compromise or allow Belfast to be treated ddifferently to London in the stop-start Brexit negotiations.

Earlier, on a visit to Newbridge, Co. Kildare, Varadkar publicly confirmed for the first time that Ireland was preparing for a no-deal Brexit which, he said, would be “catastrophic.”

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He said, “For all of us but particularly for the U.K. the consequences of a no-deal cliff edge Brexit at the end of March next year is potentially catastrophic, really bad for Ireland, relatively bad for the EU, but quite a disaster for the U.K.”

Varadkar insisted, however, that talks are still ongoing and that a previously rock-solid deadline for an October agreement is now likely to be extended to November or December.

He spoke as European Council President Donald Tusk warned in a letter to members that the worst case scenario, no deal, was “now more likely than ever before.”

British Prime Minister Theresa May, whose government is propped up by the DUP, will make a last-ditch appeal this week to EU leaders in a bid to rescue the Brexit talks which are in crisis over the Northern Ireland border.

Sinn Fein leader Mary Lou McDonald, who met May in London on Monday, said the British premier needed to show leadership.

McDonald added, “Ireland should not be the collateral damage of a Tory Brexit agenda, and if Mrs. May hitches her wagon to the most extreme Brexiteers, including the DUP, I think that history will judge her very badly.”

She warned that a hard Brexit would be followed by “an immediate demand for a referendum on Irish unity.”

“I think it's imperative that the Irish government holds its nerve, I think it's imperative that our European partners do as Michel Barnier (EU chief Brexit negotiator) promised and don’t blink.”

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British Prime Minister Theresa May.Caty Bartholomew