European Council President Donald Tusk said on Thursday the EU was "fully behind Ireland" over the latest Brexit plan and "unconvinced" by Boris Johnson's latest border proposals.
It was one of several politely sceptical reactions to the British Prime Minister's suggestion for an alternative to the Irish backstop — the mechanism required by the EU to maintain the integrity of the European customs union and single market after Brexit.
The Irish government, the European Commission president and the Brexit steering group in the European Parliament are among those to criticise the proposals.
Irish Taoiseach Leo Varadkar said they "fall short in a number of aspects."
Under the plan submitted by the British government to the EU, Northern Ireland would leave the EU's customs union along with the rest of the UK at the end of a transition period. This means there would have to be new customs checks on the island of Ireland — which Brussels and Dublin have always opposed.
Northern Ireland would stay aligned to EU single market rules for goods, but this would be subject to approval by the Belfast assembly every four years.
In the House of Commons on Thursday, the British prime minister said the government had shown flexibility and its plan was a "compromise".
But there are growing signs that major gaps remain between the British proposals and the EU's red lines, as European leaders come forward to highlight concerns.
"It's very much the view of the Irish government and people of Ireland, North and South, that there shouldn't be customs checkpoints or tariffs on trade between North and South," Irish Prime Minister Leo Varadkar told reporters.
Ireland's Foreign Minister Simon Coveneytold the Irish parliament that if Johnson's proposal was the UK's final offer, there would not be a deal.
However, he described the British plan as a "serious proposal on the table", adding that he hoped it would be "a stepping stone" towards a final agreement.
On Friday he said finding a new Brexit deal was "not mission impossible" and could succeed — as long as any new agreement also secured an open Irish border.
After meeting with Varadkar on Thursday, the Swedish PM Stefan Löfven showed support for his Irish counterpart, declaring: "We must be very firm in avoiding a hard border between Northern Ireland and Ireland."
The European Parliament's Brexit steering group, agreeing with Varadkar, said that "the UK’s proposals fall short and represent a significant movement away from joint commitments and objectives". The group added that the British proposals "do not address the real issues that need to be resolved if the backstop were to be removed, namely the all-island economy, the full respect of the Good Friday Agreement and the integrity of the Single Market".
The parliament's Brexit coordinator, Guy Verhofstadt, tweeted that the British plan was "repackaging old, bad ideas".
Using more diplomatic language, Donald Tusk echoed such concerns on Twitter, saying: "Today I had two phone calls on Brexit, first with Dublin then with London. My message to Taoiseach (Irish prime minister) Leo Varadkar: We stand fully behind Ireland. My message to Prime Minister Boris Johnson: We remain open but still unconvinced."
The European Commission President Jean-Claude Juncker said he joined the Irish prime minister in calling on the UK to publish the full legal text of its proposal.
Juncker followed up on comments he made on Wednesday, when he said he "welcomed" Johnson's "positive advances", but noted "still some problematic points" particularly concerning the backstop.
In Thursday's statement, the European Commission president called for a "legally operational solution" that could not be based on "untried arrangements that would be left to negotiation during the transition period. Accepting such a proposal would not meet all the objectives of the backstop". He called for further discussions with British negotiators.
French President Emmanuel Macron has urged both sides to continue working, to see whether a deal is possible between the UK and the EU. He stressed that an agreement would need to respect European Union principles.
The French Secretary of State for Foreign Affairs Amélie de Montchalin said after Johnson's proposals were released that "France does not want a tax haven at Europe's doorstep" and called for Brexit to "be done in an orderly manner". But she added: "If they want to do it differently, we have prepared ourselves."
The EU's chief Brexit negotiator, Michel Barnier, said he had updated the European Parliament and EU governments, and reiterated that the EU wanted "workable and effective solutions that create legal and practical certainty".