Michel Barnier, the EU chief negotiator for Brexit, on Thursday criticized the UK government for their proposals on how to deal with the post-Brexit border between Ireland and Northern Ireland, saying he was worried that "the UK wants to use Ireland as a kind of test case for a future UK-EU customs relationship."

Last month, the UK government proposed a freewheeling series of waivers for various people and goods that will cross the Irish-Northern Irish frontier after Brexit — part of their plan to avoid the return of border posts at what will become an external frontier of the EU once the UK officially exits.

The issue of Ireland and the avoidance of a hard border is one of three main priorities for the EU in the Brexit negotiations but Mr. Barnier, speaking in Brussels after the publication of the EU's 'Guiding Principles on the Dialogue for Ireland/Northern Ireland', was strongly critical of the UK's current stance in terms of customs and economic movement.

"What I see in the UK's paper on Ireland and Northern Ireland worries me," Mr. Barnier said.

"The UK wants the EU to suspend the application of its laws, its Customs Union, and its Single Market at what will be a new external border of the EU. And the UK wants to use Ireland as a kind of test case for the future EU-UK customs relations. This will not happen."

Can a hard border be avoided?

Given the UK's insistence that it intends to quit not just the EU but also the customs union and single market, Mr. Barnier, as well as the Irish Prime Minister Leo Varadkar, is skeptical about how future border checks on imports could function without a 'hard' border, namely one that is manned by customs officials. 

The Irish Government — determined to achieve as frictionless a border as possible — has long argued that the best way to avoid this would be for the UK to remain in the European Economic Area (EEA) as a non-EU member, the current status of countries such as Iceland and Norway. However, so far in the EU-UK negotiations, the UK has shown a preference for looser official economic links with the EU.

While both the EU and the UK clearly want to avoid a hard border in Ireland, Barnier's comments suggest that the EU do not believe the key watermark of "sufficient progress" has been made on the issue in the negotiations to date, meaning other major economic issues, such as the future trading relationship between the EU and the UK, are likely to remain on hold.

The four-page paper released by the EU on Thursday does not provide its own proposals on economic issues relating to the future Irish-UK border. It has repeatedly said it is up to the UK to come up with effective solutions to an issue that could yet develop into the central one of the Brexit negotiations.