Spain's Prime Minister Pedro Sanchez is suggesting this weekend's Brexit meeting of European leaders is likely to be called off, if no agreement's reached over Gibraltar.
He's has made an objection about the disputed territory, and wants changes to both the draft withdrawal treaty and declaration on future ties.
EU leaders, including Taoiseach Leo Varadkar, are supposed to be endorsing the agreement this Sunday.
It comes as DUP leader Arlene Foster warned that the deal propping up UK PM Theresa May's government would be at risk, if the Brexit deal passes in Westminster.
She told BBC Radio Ulster's Inside Politics: "If she is successful in parliament, and there is no evidence that she is going to be successful in parliament, then of course we will have to revisit the confidence and supply agreement.
"That agreement was about giving national stability, it was acting in the national interest and delivering on Brexit.
"If this is not going to deliver on Brexit then of course that brings us back to the situation of looking at the confidence and supply agreement, but we are not there yet and we are not going to jump ahead until we see what happens in parliament."
Spain has vowed to 'veto Brexit' over the Gibraltar issue, with Madrid calling for a written UK promise on the peninsula before Sunday's summit.