An internal EU paper has suggested that the avoidance of a hard border on the island of Ireland effectively requires Northern Ireland to remain in the single market and the customs union.

The working paper, seen by RTÉ News, said that in order to avoid a hard border it is essential that there be no divergence of rules on either side of the Irish border.

That scenario would effectively require Northern Ireland to remain in the single market and the customs union.

The working paper is described as an internal document from the EU's Brexit Task Force led by Michel Barnier.

It was circulated to the 27 member states on Wednesday night and presented to British negotiators in Brussels yesterday.

It comes as Mr Barnier met Brexit Secretary David Davis in Brussels today for round six of the negotiations.

The paper contains the most explicit expression yet by the EU that the only way to avoid a hard border is for the UK to remain in the customs union and the single market, or at least for Northern Ireland to remain within those structures.

It explicitly reflects the Government's thinking that the avoidance of a hard border requires much more than the kinds of technical solutions favoured by the British government.

The paper says that both the EU and UK have already committed to protecting the Good Friday Agreement. 

It says achieving this must be done in a way that respects the integrity of the internal market and the customs union, of which Ireland will remain a full member.

It then says that it seems "essential" for the UK to make sure there is no regulatory divergence from those rules of the internal market and the customs union which are, or may be in the future, necessary for meaningful north-south cooperation, the all-island economy and the protection of the Good Friday Agreement.

In other words, if Britain wants to avoid a hard border, and to protect the Good Friday Agreement, the only real way to do so is that if both parts of the island are in the customs union and the single market.

The Commission document will place more pressure on the Conservative government given its reliance on the DUP for survival.

Northern Ireland remaining inside the customs union and single market, while the rest of the UK was outside, would impose an entirely new structure on the United Kingdom.

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The DUP has consistently warned it will not accept anything that undermines the union between Northern Ireland and Great Britain.

For the EU to adopt this stance now suggests that it expects the UK to come around to its position if it wants to move to Phase II of the Brexit negotiations, which deals with the future trading relationship.

The EU has said the UK can only qualify to that next phase if there is "sufficient progress" on the financial settlement, EU citizens' rights, and the Irish border.

A deadlock in the negotiations meant there was no breakthrough at the summit of EU leaders on 14 and 15 October.

While the UK's exit bill has been widely seen as the most difficult stumbling block, one EU source has told RTÉ News that the Irish question may turn out to be an even more difficult issue as the deadline of the December EU summit approaches.

On Monday, the Secretary of State for Northern Ireland James Brokenshire said in a speech in Brussels that it would be "impossible" for Northern Ireland to remain in the customs union and single market, while the rest of the UK remained outside.

He did suggest there could be some aspects of agrifood that were handled on an all-island basis, but Irish sources say he did not fully explain how far that would go, or how it would avoid a hard border.

The task force working paper reveals that both the EU and UK have reached an agreement in principle on the Common Travel Area.

"At the last round [of negotiations], agreement was reached in principle on joint principles on the Common Travel Area, which aim to recognise an existing bilateral arrangement between the UK and Ireland," the paper states.

But its articulation of the customs union and single market issues are likely to prompt fierce resistance from the DUP.

Another senior EU official told RTÉ News that the DUP's role in propping up Theresa May's government was threatening to thwart the kinds of "flexible and imaginative" solutions to the border dilemma that the EU has been settling upon.

Irish sources say the task force working paper "captures" Ireland’s long-held view, which has been expressed on several occasions by Taoiseach Leo Varadkar and Minister for Foreign Affairs Simon Coveney, that the only real way to avoid a hard border is for the UK to remain in the customs union.

Officials say this is not a hardening of the EU’s position, but merely the logical outworking of the EU27’s beliefs as expressed as recently as the October European Council, but also in the EU’s Negotiating Guidelines.

They say that a hard border can only be avoided by "meaningful" solutions put forward by the UK.