European Council President Donald Tusk has said that if the UK offer on the border on the island of Ireland is unacceptable for Ireland, then it will also be unacceptable for the EU.
Mr Tusk made the comments after meeting with Taoiseach Leo Varadkar in Dublin.
A former Polish prime minister, Mr Tusk will chair a crunch summit of European Union leaders on the issue in two weeks.
"Before proposing guidelines on transition and future relations to the leaders, I will consult the Taoiseach if the UK offer is sufficient for the Irish Government," Mr Tusk said.
"Let me say very clearly: If the UK offer is unacceptable for Ireland, it will also be unacceptable for the EU. I realise that for some British politicians this may be hard to understand," he said.
"This is why the key to the UK’s future lies - in some ways - in Dublin, at least as long as Brexit negotiations continue."
Mr Tusk noted that he had given British Prime Minister Theresa May a deadline of Monday to make a "final offer" on the border before leaders decide whether there is "sufficient progress" on a divorce settlement to merit opening talks on the future relationship.
He insisted there could be no division between the other EU member states and Ireland: "The EU is fully behind you and your request that there should be no hard border on the island of Ireland after Brexit," he said.
"The Irish request is the EU’s request."
He added: "The UK started Brexit, and now it is their responsibility to propose a credible commitment to do what is necessary to avoid a hard border."
"On the question of the border, as I have said many times, the best and most obvious solution would be for the UK to remain in the Customs Union and the Single Market," said Mr Varadkar.
"But if the British government continues to rule out that option, it must offer credible, concrete and workable solutions that guarantee there will be no hard border, whatever the outcome in the negotiations, and whatever the future relationship between the EU and the UK," added Mr Varadkar.
Mr Varadkar said Ireland appreciated the solidarity from its EU partners.
"They have taken Ireland's unique concerns to heart - these are seen very much as EU concerns.
"It is an important and impressive display of EU unity and solidarity, the European Union is a family and we stick together."
Talks between the UK and the EU have intensified ahead of next Monday's meeting and the 14 December summit.
MPs at Westminster say British ministers have failed to explain how a return to border controls can be avoided if the UK leaves the EU Single Market and the Customs Union.
The House of Commons committee for exiting the EU said last night that the British government's proposals for resolving the border issue, such as the use of technology, are untested.
Coveney: Ireland cannot be expected to take a 'leap in the dark'
Earlier, Tánaiste and Minister for Foreign Affairs Simon Coveney said Ireland's position on the border issue has not changed and it cannot be expected to take a "leap in the dark" and move on to the second phase of Brexit negotiations.
Speaking on BBC Radio 4 this morning, Mr Coveney said the attitude of the British government is "we will solve this problem, but not right now".
"What the British government is asking of the Irish Government is, ‘trust us we will solve these issues with a broad bold trade agreement’ and that may not be possible.
"We can't be asked here to leap into the dark by opening up phase two discussions."
Mr Coveney said he hopes there will be a successful outcome within the next two weeks.
He said Ireland wanted to achieve "an agreed wording whereby we can agree the parameters within which we can find a solution that prevents the re-emergence of a border on the island of Ireland" before a 14 December EU summit.
He added that he continues to try to avoid the Brexit debate becoming an issue of "green versus orange" in Northern Ireland.
However, he would not comment on recent statements from senior DUP members, instead saying they want to work with all parties in Northern Ireland.
Former DUP leader Peter Robinson has accused the Irish Government of acting provocatively in the negotiations and said the South should "wind its neck in".
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On Friday, British media reported the DUP warned Mrs May's Conservative Party against giving too many concessions in the talks on the future border arrangements.
It was reported that DUP officials met the Tory officials on Friday morning, when they warned that any British commitment on Northern Ireland staying in the single market, customs union or regulatory equivalent "would be deeply destabilising" for the deal they struck earlier this year to keep Mrs May in power.
Mr Coveney said no party has a monopoly on what is acceptable, and he assured Unionists that the Brexit negotiations are not an effort by the Government to affect the constitutional position of Northern Ireland.