Brussels (CNN) Britain and the European Union failed to reach a highly anticipated Brexit deal on Monday despite "significant progress" on key outstanding issues, both sides announced in Brussels.

Negotiators had earlier appeared close to reaching an agreement on the Irish border, the complex and historically sensitive issue that had emerged as a final stumbling block.

But after details of a draft deal leaked, the Democratic Unionist Party, a small but hardline grouping from Northern Ireland that props up the minority government of British Prime Minister Theresa May, said they would not support it.

After talks over lunch, May and EU Commission President Jean-Claude Juncker acknowledged that some issues remained unresolved. Both said they were confident of a deal soon that would allow talks to progress to a future trading relationship.

Juncker said that it was not possible to reach a deal on Monday despite "significant progress" made in recent days. "This is not a failure, this is the start of the very last round. I am very confident that we will reach agreement in the course of this week," he said.

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