Significant gaps in Theresa May’s new Brexit customs plan for Northern Ireland would lead to the creation of a hard border with the Republic, the European Commission has said.

A presentation drawn up by the EU’s negotiating team for member states says the UK’s customs paper, which was released on Thursday, “leaves key questions unanswered”.

Officials are particularly worried that the UK plan would not keep Northern Ireland aligned with single market rules, meaning regulatory checks would be needed at the border for goods.

They are also concerned at UK proposals to time-limit the proposal until 2021 at the latest, with the presentation asking: “Is this a backstop?”

The presentation concludes that the UK proposal “does not cover regulatory controls, leading to a hard border”.

Brexit Secretary David Davis met with chief negotiator Michel Barnier in Brussels on Monday to discuss progress in the negotiations. The trip is only Mr Davis’s second visit to the de facto EU capital this year, with progress in talks having been slow. The minister declined to speak to the press during the trip.

The so-called backstop solution is supposed to prevent a hard border from emerging on the island of Ireland no matter what happens. Both sides say they do not want a hard border because the open frontier is part of the Good Friday Agreement, which brought an end to the Troubles.

Mr Barnier last week said he was still studying the UK’s proposal and had not rejected it out of hand. When asked why he was doing so if it has such problems, he said: “Because I look at all British proposals in an objective manner.”

The UK paper is the product of months of Cabinet wrangling over how to temporarily solve the Irish border issue if it cannot be solved by a wider trade deal.

Separately, Cabinet has still not decided on a single solution for the wider customs agreement between the UK and EU – with two options on the table. Both ideas, “MaxFac” and “customs partnership” have been rejected out of hand by Brussels.

A spokesperson for the European Commission said: “We can confirm that the European Commission chief negotiator Michel Barnier and the Secretary of State for Exiting the European Union David Davis met this morning here in Brussels where they discussed the latest state of play of the Article 50 negotiations. They touched on issues related to the withdrawal agreement, including the backstop for Ireland-Northern Ireland and the framework for the future relationship.” British officials confirmed a similar line; neither side would give more details.

Officials on both sides are getting ready for a European Council summit in March where member states will take stock over the state of negotiations so far.