THE taoiseach has warned that any attempt by Britain to delay Brexit beyond the end of October would face "enormous hostility" from the European Union.
Speaking before a European Council summit in Brussels, Leo Varadkar said there will be "ongoing European unity" regardless of the new British Prime Minister.
And he said the Brexit deadline would not be extended for further negotiations or indicative votes, despite claims from some senior Tories that they will be able to make changes to the withdrawal deal.
"Any negotiations will have to happen between the UK and the EU," he said.
"We’re not going to allow the UK to tour the capitals and try to engage in bilateral agreements. That’s not how this works.
"The Withdrawal Agreement is not going to be reopened but we will consider amendments to the joint Political Declaration and if there is no Withdrawal Agreement there will be no transition period for the UK."
Mr Varadkar said the British Prime Minister must recognise that the planned withdrawal agreement will not be renegotiated and "a basic fact that if we don’t have an agreement by October 31, there won’t be an extension other than for a referendum or an election".
He added: "While I have endless patience, some of my colleagues have lost patience, quite frankly, with the UK, and there's enormous hostility to any further extension.
"So, I think an extension could only really happen if it were to facilitate something like a general election in the UK or perhaps even something like a second referendum, if they decided to have one.
"What won't be entertained is an extension for further negotiations or further indicative votes. The time for that has long since passed."
The taoiseach was speaking after a major new document found that north-south co-operation will be seriously affected by a hard border or customs checks.
The scale of all-Ireland co-operation has been mapped out in the European Commission document, covering more than 100 areas from health, transport and agriculture to the preservation of Lough Neagh's eels.
The paper found that "virtually all areas of north-south cooperation are predicated on the avoidance of a hard border, including related customs or regulatory checks and controls".
The report said that good working relations between north and south were a "central part of the Good Friday Agreement and is essential for achieving reconciliation and the normalisation of relationships on the island of Ireland".
According to the commission, emergency health co-operation could be hit by delays at the border.
The UK government published its version of the mapping exercise in December.
Meanwhile, the British government has appointed a panel of experts to advise on how to maintain a frictionless border after Brexit.
The panel includes PSNI assistant chief constable Tim Mairs and academic Katy Hayward from Queen's University, Belfast.
Representatives from business groups in the north will also give their input.
The panel will be chaired by Brexit Secretary Steve Barclay and Financial Secretary to the Treasury Jesse Norman.
