The Irish government has flatly rejected a leaked version of Boris Johnson's Brexit plan for the Irish border, ahead of its official unveiling on Wednesday.

Leaks reported by the Daily Telegraph suggest the UK government is proposing a plan that would not prevent customs checks between Ireland and Northern Ireland, and also be time-limited – two longstanding EU red lines.

Simon Coveney, Irish deputy prime minister, said the reports of what the UK was about to propose were "concerning to say the least".

He was joined by EU officials and diplomats in Brussels, who dismissed the proposals as "fundamentally flawed".

"If it's take it or leave it, we better close the book and start talking about the modalities of an extension," one senior EU official said.

Another diplomat added: "This won't fly. Johnson has chosen a confrontation. The remaining options are the original, Northern Ireland-only backstop with some modifications, or an extension."

Irish deputy PM Mr Coveney said he had not been shown anything officially, but said: “We’ll have to wait and see. Obviously we’ll study any proposal carefully, but if the reports are true it doesn’t look like the basis of an agreement, that’s for sure.

“Our position has been consistent, respectful and clear. If there is to be an alternative to the backstop it has got to do the same job as the backstop, which means no physical border infrastructure on the island of Ireland, and no related checks or controls.

“The idea that you would be putting a customs border effectively between Northern Ireland and the Republic is not something that is consistent with that, or even close.”

Irish deputy prime minister Simon Coveney (PA)

The Tánaiste also cast doubt on whether the UK was serious about finding a deal, given the tin-eared nature of the proposals coming out of London.

“If there is a proposal that involves customs checks on the island of Ireland, that in itself is bad faith given the commitments the British government has given both to Ireland and the EU over the last three years," the deputy prime minister added.

Mr Johnson is expected to tell Conservative party conference on Wednesday afternoon that the plans are a "final offer" to the EU to avert a no-deal in October 31. The formal proposals are expected to be sent to Brussels after the close of his speech.

UK government officials have not questioned the fundamentals of the leaks – though they dispute unspecified "elements". The UK side is blaming Dublin for making the plans public ahead of Mr Johnson's speech.

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Asked whether the plan would fly, Helen McEntee, Ireland's Minister for European Affairs, told Irish broadcaster RTE: "No. What we're talking about again is picking and choosing certain parts of the single market that would be aligned in Northern Ireland. It's talking about a time limit, which again is not acceptable, but it's still talking about the need and the requirement for customs checks.

"Again, to go back to the commitments, it's no infrastructure or associated checks, protecting the Good Friday agreement and the areas of cooperation, protecting the all-Ireland economy, and protecting the single market and our place in it. If that's what's being proposed it certainly doesn't do any of that."

The British proposals again managed to unite Ireland's political parties in condemnation. Lisa Chambers, Fianna Fáil Spokesperson on Brexit told the same programme: "The entire Oireachtas [Irish parliament] supports that position. If that's actually the proposal from the UK government you'd really have to question is there a sincere effort to get a deal done.

"I think a deal can be done, and I think that Boris Johnson is running out of options. But if they actually put those proposals on paper to Brussels and support a time-limited backstop just for regulations and customs checks immediately, they know that won't wash, they know that won't be accepted. You have to ask the question are they putting those proposals there knowing they're going to be rejected because they're pursuing a no-deal policy?"