The future of the border between the Republic of Ireland and Northern Ireland is "particularly crucial" for the EU, but the decision whether Brexit talks can enter a second phase will depend on the assessment of Michel Barnier, Dutch prime minister Mark Rutte told EUobserver.

"This is about the stability of the island as a whole, the Republic and Northern Ireland. This is about the border between UK and the whole European Union," he said.

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Rutte spoke to this website in the margins of the Alde party congress in Amsterdam, a day after European Council president Donald Tusk said in Dublin that "if the UK's offer is unacceptable for Ireland, it will also be unacceptable for the EU."

The Irish government is asking guarantees from the UK that Brexit will not create a 'hard border' on the island, which would disrupt the economy and jeopardise the Good Friday peace agreement.

Rutte insisted however that EU leaders, at their summit on 14-15 December, "will take the decision collectively, based on what our negotiator, Michel Barnier, will tell us."

He said that the decision to go into phase two of Brexit negotiations, which would be about the future EU-UK relationship and a possible transition period after Brexit, "will depend on [whether] enough progress [has been made] on three themes: the exit bill, the citizens rights, and the border question."

Barnier, an 'excellent negotiator'

"Ireland seems to be the most difficult issue. But it is good for the 27 leaders to refer to the chief negotiator," he insisted.

"He's an excellent negotiator, I have the highest trust in him," the Dutch PM added.

On Monday (4 December), Barnier and European Commission president Jean-Claude Juncker will receive British prime minister Theresa May in Brussels for what appears like a last chance meeting before the EU summit.

Barnier will brief the college of commissioners on Wednesday, before issuing its recommendation to member states. EU advisers of the 27 leaders will meet on 11 December to try to agree on their common position ahead of the summit three days later.

The Dutch PM said that he was "an optimist" and he hoped that "the EU will be able to say that there is sufficient progress."

But he pointed out that he could "not talk about it openly" on how talks are going, "because others would be reacting to what I'm saying to you and that could harm somehow the possibility to get to a successful outcome."

He argued that what he would "guess or think based on [his] conversations with Theresa May and others … might just be slightly different from what somebody else is saying, and that would diminish the possibility for the 27 to stick together and that's why we always refer to Michel Barnier."

No progress, no phase two

Asked whether phase two could wait, Rutte said "hopefully not."

"But if there is not sufficient progress, there will not be a phase two," he warned, sticking to the position expressed by all EU leaders.

While May is under pressure from the EU to give guarantees to Ireland, she is also being pressed by Northern Ireland unionists not to give ground.

If the UK government is "prepared to have Northern Ireland treated differently than the rest of the UK, then they can't rely on our vote," said Sammy Wilson, an MP from the Democratic Unionist Party (DUP), on whose support May's government depends in the House of Commons.

Asked whether the DUP would prevent May from giving the EU the guarantees it is asking, Rutte said he did not want to "speculate about internal British politics."

"There is a good tradition that we try to stay away from that as much as possible," he noted, before adding: "It's difficult for me to predict what is happening in Dutch politics, let alone what is happening in London or Belfast."