Brussels has reacted with “concern” to aspects of Boris Johnson’s proposal for the Irish border – warning that it will have to change.

Commission president Jean-Claude Juncker told Mr Johnson in a phone call that the proposal contained “problematic points”, while an EU source warned that the bloc’s parliament would “never support” the plan.

Meanwhile Irish prime minister Leo Varadkar told Mr Johnson in a separate phone call that the plans “do not fully meet the objectives of the backstop”.

Guy Verhofstadt, the European Parliament’s Brexit coordinator, was even more scathing, telling reporters in Brussels after a meeting with Michel Barnier that the plan was not good enough.

“I can tell you that the reaction of the Brexit steering group was not positive, not positive in the sense that we don’t think this is really the safeguard that Ireland needs,” he said.

“But tomorrow we will give point after point the reasons for that, what is not acceptable in the proposals.”

He also suggested that the proposals might have been offered in an attempt by the UK to try to blame on the EU side for a no deal.

“There’ll be a statement saying why European Parliament will never support this tomorrow,” a source said.

EU leaders have, however, said they will engage with the proposal and study it in further detail – after Mr Johnson dropped an earlier stipulation that the proposal was his “final offer”.

Under the proposal, goods moving between Ireland and Northern Ireland would be subject to customs checks, but not regulatory checks – while goods moving between Great Britain and Northern Ireland would be subject to regulatory checks but not customs checks.

The UK plan suggests that checks could “take place at traders’ premises or other designated locations which could be located anywhere in Ireland or Northern Ireland”.

The EU side is concerned about the customs border with the Republic, as well as “governance” issues – in particular a stipulation that the Stormont assembly can veto the plan after four years.

Boris Johnson speaking at Tory conference earlier on Wednesday (AFP)

Behind the scenes, British officials are arguing that Northern Ireland cannot be kept in a backstop arrangement against its will and so must be given a vote every four years.

The UK plan would also require the EU to rip up aspects of its customs rules to suit the UK plan. Asked if the proposal meant the UK expected the EU to change its customs rules, a senior UK official said: “We are asking them that because we agreed there will be flexible and creative solutions on the island of Ireland and part of it is adjusting legislation to enable it.”

A European Commission spokesperson said: “President Juncker welcomed prime minister Johnson’s determination to advance the talks ahead of the October European Council and make progress towards a deal.

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“He acknowledged the positive advances, notably with regards to the full regulatory alignment for all goods and the control of goods entering Northern Ireland from Great Britain. However, the president also noted that there are still some problematic points that will need further work in the coming days, notably with regards to the governance of the backstop.

“The delicate balance struck by the Good Friday Agreement must be preserved. Another concern that needs to be addressed is the substantive customs rules.

“He also stressed that we must have a legally operational solution that meets all the objectives of the backstop: preventing a hard border, preserving north-south cooperation and the all-island economy, and protecting the EU’s single market and Ireland’s place in it.”