The European Union is "open but not convinced" by the UK PM's new proposals for a Brexit deal with the EU, the president of the European Council has said.
Donald Tusk said on the EU was "fully behind Ireland" over the latest Brexit plan and had doubt over Boris Johnson's withdrawal agreement plan.
The European Parliament's Brexit Steering Group (BSG) said it has "grave concerns" about the proposals, which it said cannot be backed "in their current form".
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.
The Irish government has also raised objections to the post-Brexit arrangements proposed in Johnson's plan. Irish prime minister Leo Varadkar said he could not fully understand how the UK saw Northern Ireland and the Republic operating under different customs regimes without the need for customs posts.
Ireland's foreign minister Simon Coveney told 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.
The EU's chief negotiator, Michel Barnier, has told European diplomats he still has plenty of questions about the British proposal to replace 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". Jean-Claude Juncker called for further discussions with British negotiators.
On Thursday, Johnson said he had made a "genuine attempt to bridge the chasm" with EU officials before time runs out to reach a deal for the 31 October deadline for the UK to leave the EU.
Jeremy Corbyn said no Labour MP could support the "reckless deal" that he said would threaten the Good Friday Agreement, which secured peace in Northern Ireland.
The UK government said it is aiming to reach a final agreement at an EU summit on 17 October.