Brexit deal more likely in November or December: Irish PM

"I know some people are optimistic about an agreement on the withdrawal agreement protocol this week. I have to say I always thought that was unlikely," he told reporters.

"I figure November, December is probably the best opportunity for a deal, but this is a dynamic situation."

Varadkar will travel to Brussels for the summit taking place on Wednesday and Thursday, where UK and EU officials had hoped to lay out details of the Brexit withdrawal plan.

But both sides remain locked in a stalemate over the question of the Irish border -- and how to keep it open once Britain leaves the EU's single market and customs union.

Under EU proposals, the British province of Northern Ireland would follow the bloc's trade rules unless and until a more comprehensive trade deal is forged. But British Prime Minister Theresa May has said this so-called "backstop" plan would divide the United Kingdom, instead proposing a UK-wide "temporary customs arrangement".

The EU and Ireland have insisted that the backstop must be permanent to be a binding assurance against a hard border.

"There are some fundamentals that we can't compromise on," Varadkar said.

"As part of that withdrawal agreement there must be a legally operable and legally binding assurance that no matter what happens that no hard border will emerge on the island of Ireland."

Varadkar will meet later Monday with Arlene Foster, the head of Northern Ireland's Democratic Unionist Party (DUP), which props up May's government in London.

Foster has threatened to withdraw her support if May allows any controls to divide Northern Ireland from mainland Britain.

She has also previously made the controversial statement that the 1998 Good Friday peace agreement is not "sacrosanct" -- implying it could be altered to solve the deadlock.

The accord, which guarantees free movement between the north and the Republic, is widely credited with bringing peace to the border and its preservation is a key priority for Dublin.