Ireland's deputy prime minister has said his country "cannot possibly" support Boris Johnson's Brexit plan, and encouraged the UK to come back with something "fit for purpose".

Speaking in the country's parliament on Thursday Simon Coveney echoed EU concerns about customs checks and a built-in veto for the Northern Ireland Assembly that are included the policy.

"We cannot support any proposal that suggests that one party or indeed a minority in Northern Ireland could make the decision for the majority in terms of how these proposals would be implemented in the future," he told the Dáil.

"That is not consistent with the Good Friday Agreement. It is not something we could possibly support as part of any final deal."

The Tánaiste added that there were "legal and technical" problems with the plan, stating: "Despite this paper saying they want to avoid customs checks they do raise the prospect of customs checks somewhere, not just in premises and businesses, and we think that's going to be a real problem."

Mr Coveney expressed hope that the plan could be developed into something that was "fit for purpose" and be a starting point for negotiations. On Wednesday night Leo Varadkar said the proposals did not meet the objectives of the backstop to prevent a hard border.

The deputy PM's comments come as the European Commission warned that it was down to the UK to fix the "problematic" aspects of the proposals – which in the eyes of Brussels are more-or-less the same as those identified by Dublin.

Asked whether Brexit Secretary Steve Barclay was right to say that the ball was "in the EU's court" following the release of the policy on Wednesday, 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.

Boris Johnson's plan was dismissed by Ireland's deputy prime minister, Simon Coveney. (AP)

"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.

It is not something we could possibly support as part of any final deal. Simon Coveney, Ireland's deputy prime minister

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".

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 – with critics warning the sites would be customs centres in all but name.

EU says ball is still in the UK's court on getting Brexit deal done

The Northern Ireland assembly and executive would also have to vote to keep the plan going every four years, effectively constituting a veto.

The proposal has been received very poorly in Northern Ireland, where only the DUP has welcomed it out of the territory's main political parties. Business groups also condemned the plan, with a spokesperson for Manufacturing NI saying it would be "worse" than a no-deal Brexit and "decimate" entire industries.

In the Commons, Boris Johnson indicated that he might be open to making further concessions to Brussels if it helped unblock talks. But he also faced a torrent of criticism that his plan lacked support in Northern Ireland and would break the law by imposing new customs checks.

Challenged to name a single business that backed the blueprint, the prime minister was only able to say that companies badly wanted an agreement of some sort.

Hilary Benn, the chairman of the Commons Brexit committee, protested that the proposals put the Good Friday Agreement at risk.

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He pointed out that the withdrawal act passed last year bars any new “checks and controls” in Ireland – yet Mr Johnson had announced “there will be customs checks in Northern Ireland”, albeit not at the border.

And Sylvia Hermon, Independent MP for North Down, said: “The prime minister's proposals prove quite clearly that he does not understand Northern Ireland.

"While the prime minister is perfectly happy, it seems, to dance to the tune of his friends in the Democratic Unionist Party, he forgets or chooses to ignore the fact that the DUP does not represent the majority of people in Northern Ireland.

Lady Hermon added: “The majority of people in Northern Ireland will be extremely concerned by the proposals that he has tabled yesterday, which introduce two borders