THE IRA’s political wing Sinn Fein is to blame for a serious new Brexit standoff over Northern Ireland’s border, ministers say.

A leaked Brussels document yesterday revealed the Irish Taoiseach has significantly hardened the EU’s line over how to keep the border with the south open after Britain’s EU exit.

Sinn Fein's president Gerry Adams has put pressure on the Irish PM to adopt the position Credit: Reuters

In an ‘all island’ solution, Leo Varadkar is now insisting Ulster remain part of the single market and customs union while the rest of Britain leaves it.

Ministers have told The Sun that the Irish PM has only been forced to adopt the stand under heavy political pressure from Gerry Adams’ Sinn Fein.

His Republican party are set to make big gains in new elections to the Dail, which could some soon if Mr Varadkar’s weak Fine Gael government collapses.

Any separate arrangement for Northern Ireland was adamantly shot down by Brexit Secretary David Davis yesterday as an impossibility.

Ireland's Leo Varadkar said on Friday that Northern Ireland may be able to remain in the Single Market Credit: AP:Associated Press

Saying he had “frank discussions” about the new proposal with EU negotiator Michel Barnier in Brussels, Mr Davis said: “We respect the European Union desire to protect the legal order of the single market and Customs Union.

“But that cannot come at cost to the constitutional and economic integrity of the United Kingdom.

“This cannot amount to creating a new border inside our United Kingdom.”

Senior Tories last night warned the Irish demand risks sinking the whole Brexit negotiation.

Former Cabinet minister and DexEU committee member John Whittingdale said: “If the Taoiseach takes the view that protecting his political flank is his priority, he must understand that there will be no deal.

“Sinn Fein should not be trying to profit from making a Brexit deal very hard to reach.”

But pro-EU Labour MP and Open Britain supporter Conor McGinn said: “The only way to avoid a hard border on the island of Ireland, or a new one between Northern Ireland and the rest of the UK, is for the British Government to change course and negotiate to keep the whole of the UK in the Customs Union and the Single Market for the long term.”