Brussels hardened its rhetoric against Boris Johnson's Brexit plan on Thursday, warning that it is up to the UK, not the EU, to fix "problematic" aspects of it before negotiations can start in earnest.

Asked whether Brexit Secretary Steve Barclay was right to say that the ball was "in the EU's court" following the release of the proposals, a spokesperson for the European Commission said the EU would not be left "holding the bag" and that it was the UK that needed to act.

"We would disagree." she told reporters in Brussels. "There are, as we have said, problematic points in the United Kingdom's proposal and further work is needed – but that work needs to be done by the United Kingdom and not the other way around.

"We would remind you that it's the UK leaving the European Union and not the EU leaving the UK.

"We are doing everything in our power to ensure that exit is on an orderly basis and we are willing to engage constructively with our counterparts. But we are not going to be the ones left holding the bag, the ball, or any other kind of object."

An initial polite but firm response from Brussels to the plans on Wednesday evening had been interpreted ambiguously back in the UK, with EU officials saying they would study the plans before commenting in detail.

But reaction behind the scenes hardened on Wednesday. One senior EU officials said the UK proposals "can't fly" and that plans to give the Northern Ireland assembly a veto over the plan were not acceptable.

Jean-Claude Juncker, the European Commission president, is expected to call Taoiseach Leo Varadkar on Thursday afternoon to discuss the proposals. A spokesperson for Mr Juncker said he would "reiterate the EU's continued unity and solidarity behind Ireland".

Both Ireland and Brussels have dismissed reports in the UK media that Ireland is under pressure to accept the proposals. The country's European affairs minister Helen McEntee said: "Our EU partners have stood beside us for the last three-and-a-half years, and that has not changed. We are not coming under pressure to change those key objectives."

She added that the proposals were "a basis for discussions" but that there "are obvious concerns".

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Under the British plan, Northern Ireland would stay aligned with the EU single market regulations for goods, but stay in the UK customs zone. The result would be customs checks on products moving between the Republic of Ireland and Northern Ireland, and regulatory checks on products moving between Great Britain and Northern Ireland. The UK government says the checks could be done away from the border, though has provided little detail on how. The Northern Ireland assembly and executive would also have to vote to keep the plan going every four years.