Theresa May has been warned the EU will back Ireland if it chooses to veto progress on Brexit talks over concerns about the Irish border.
European Council President Donald Tusk said if the UK's offer was unacceptable to Ireland then it would also be unacceptable for the rest of the EU states.
Ireland has warned that it is prepared to use its veto to prevent the start of trade and divorce talks unless it gets assurances that there will not be a hard border between Northern Ireland and the Republic.
Today, Mr Tusk made it clear that the bloc's loyalties lie with the Irish Taoiseach as he met with Leo Varadkar in Dublin.
Meanwhile, Mr Varadkar warned: "I am also prepared to stand firm if the offer [from the UK] falls short." He said the next couple of days will be crucial.
It comes as Mrs May prepares to travel to Brussels in the hope of kick-starting the next stage of talks - including discussions on a future free trade deal - before the year's end.
However to do that they must convince the EU that "sufficient progress" has been made on preliminary issues including the issue of Ireland's border.
All 27 EU states including Ireland will have the ability to veto the start of the second phase of talks, meaning that Mrs May must be sure of support from Dublin for progress to be made.
So far, there has been no firm proposal on how the border issue will be solved when Norther Ireland remains in the EU while Northern Ireland is out.
The open border has been critical in helping to quell the violent legacy of the troubles and also boosts trade between both countries.
The UK is pressing for a frictionless frontier on its only land border with an EU state. It wants to use technology and trading arrangements to ensure the free flow of goods and no return to the heavily militarised border of the conflict.
Ireland's demand for no checkpoints could also see Northern Ireland sticking to the EU rulebook while the rest of the UK diverged, but that is a red line for the Democratic Unionists who are propping up the UK Government.