Boris Johnson’s final “take it or leave it” Brexit offer to Brussels is in danger of being dead on arrival after it was rounded on by government and opposition parties in Ireland.

The prime minister’s proposals were described as unworkable, unacceptable and illegal under British domestic law, which bans any new infrastructure on the Irish border that did not exist before Brexit day.

“If this is the final offer, then there is not a deal to be had,” said one EU official.

Ireland’s European affairs minister, Helen McEntee, said the plans would not be acceptable to Dublin and raised doubts that Johnson was sincere in wanting a deal.

She said: “What we are talking about again is picking and choosing certain parts of the single market that would be aligned in Northern Ireland. It is talking about a time limit, which again is not acceptable.”

Johnson’s plan to seal a Brexit deal in the next 10 days will be delivered to Brussels on Wednesday with a message that the EU can “take it or leave it” as this is his final offer.

His proposal, which has the full backing of the Democratic Unionist party, has been labelled the “two borders for four years plan”.

It involves the the UK leaving the EU on 31 October with a transition period as originally envisaged until the end of December 2020, but with the vexed issue of the Irish border being solved by a specific proposal for Northern Ireland.

Quick Guide Why is the Irish border a stumbling block for Brexit? Show Counties and customs Inside the EU, both Ireland and Northern Ireland are part of the single market and customs union so share the same regulations and standards, allowing a soft or invisible border between the two.

Britain’s exit from the EU – taking Northern Ireland with it – risks a return to a hard or policed border. The only way to avoid this post-Brexit is for regulations on both sides to remain more or less the same in key areas including food, animal welfare, medicines and product safety. The 'backstop' in Theresa May's Withdrawal Agreement was intended to address this - stating that if no future trade agreement could be reached between the EU and the UK, then rules and regulations would stay as they are. This has been rejected by Brexit supporters as a 'trap' to keep the UK in the EU's customs union, which would prevent the UK striking its own independent trade deals. There are an estimated 72m road vehicle crossings a year between Northern Ireland and the Republic of Ireland, and about 14% of those crossings are consignments of goods, some of which may cross the border several times before they reach a consumer. Brexit supporters say this can be managed by doing checks on goods away from the border, but critics say it will be difficult to police this without any physical infrastructure like border posts or cameras, which could raise tensions in the divided communities of Ireland.

Interactive: A typical hour in the life of the Irish border Photograph: Design Pics Inc/Design Pics RF

Northern Ireland would remain in regulatory alignment with the EU until 2025 but come out of the customs union with the rest of the UK.

It proposes that at the end of 2025 Northern Ireland in conjunction with Ireland would decide on the level of regulatory alignment through a Belfast-Dublin “bilateral lock” in a British-Irish ministerial council.

This would mean customs checks between Northern Ireland and the Republic from 1 January 2021, something unacceptable to Dublin and DUP’s rivals in Northern Ireland.

The DUP leader, Arlene Foster, said she hoped the EU would take the offer seriously and that she wanted to “focus on a deal”.

But Lisa Chambers, Brexit spokesperson for Fianna Fáil, described the plans as “unacceptable and unworkable”.

She tweeted on Wednesday: “UK is reneging on commitments made to citizens on the island of Ireland that there would be no hard border. This would set our country back years. You would have to question if PM Johnson is genuinely serious about getting a deal, this suggests not.

“We need to see something that is credible and if this is what is being proposed it certainly won’t be acceptable not only to the Irish government but to the EU as a whole.”

McEntee also suggested the proposals, which will be presented as a fair and generous offer, reneged on previous commitments by the UK to ensure no hard border on the island of Ireland.

“It is still talking about the need and requirement for customs checks. So, again, to go back to the commitments it is 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. And if that is what is being proposed it certainly doesn’t do any of that,” she said.

The former Northern Ireland secretary Lord Hain said the Brexit plan could break the law and “sabotage” the Good Friday agreement.

The Labour peer said by insisting on extra customs checks and different trading relationships across the Irish border the government “is proposing to break the law”.

He said it would breach Britain’s own Withdrawal Act 2018, which includes a detailed clause specifically banning “border arrangements between Northern Ireland and the Republic of Ireland after exit day that feature physical infrastructure, including border posts, or checks and controls, that did not exist before exit day”.

Hain added: “This will also sabotage the Good Friday peace agreement.”

Britain’s Brexit Act passed by the government includes a ban on the very customs checks proposed by Johnson, says Lord Hain Photograph: screengrab/Withdrawal Act 2019 on Irish border infrastructure

It is understood that Dominic Cummings, the prime minister’s strategist, has warned top advisers that the UK is prepared to walk away if the EU does not engage seriously with the offer.

“To be clear, we won’t be hanging around waiting for them to negotiate with us,” he told a meeting of senior advisers. “If they reject our offer, that’s it.”

EU sources said the outlines of the proposal did not meet the objectives of the Irish backstop, and threatened both the integrity of the single market and peace in Northern Ireland. “If this is the final offer, then there is not a deal to be had,” said one EU official.

Q&A What is a Northern Ireland-only backstop? Show The British government’s version of Brexit involves the UK ultimately leaving the single market and customs union, requiring the return of a range of checks on goods crossing the border between Northern Ireland and the Republic of Ireland. The “backstop” is intended as a standstill placeholder to ensure such checks do not have to be imposed between Brexit happening with a deal, and the start of a new free trade agreement yet to be negotiated between the UK and the EU. Theresa May's withdrawal agreement proposed keeping the whole of the UK in a shared customs territory with the EU during this period. An alternative idea involves only Northern Ireland staying in the EU’s customs territory. That would place a customs border in the Irish Sea. May described it as a threat to the constitutional integrity of the UK, but the new prime minister, Boris Johnson, has opened the current talks by proposing an all-Ireland agri-food zone. The suggestion is that he will seek to quietly build on that with further NI-only arrangements. Given an NI-only backstop was an EU proposal in the first place, the U-turn would be warmly welcomed in Brussels, although attempts to give the Northern Ireland assembly a veto on its continuation would not be acceptable, and the DUP would be unlikely to support the prime minister in such a move in parliament. If there is a no-deal Brexit, then there is no backstop. Daniel Boffey

A senior EU diplomat said the two negotiating sides were in fundamental disagreement, and that there was insufficient time to bridge the gap with just three weeks to go before the UK is due to leave the bloc.

He added: “There is a difference in the starting point. We want to avoid the border and checks and controls. And the other side are saying, ‘No we will need some kind of controls, one way or another’, which is difficult for us to accept as it runs into the clear goal of the stability of the island of Ireland.”

Philippe Lamberts, a member of the European parliament’s Brexit steering group, said there was no confidence that Johnson could even guarantee the Commons’ support for his proposals.

He said: “We are in a worse situation than we were facing Theresa May because at least Theresa May had a working majority … The last thing the EU wants to do is to make concessions that won’t fly in Westminster.

“My interpretation of Boris Johnson’s actions since being prime minister is to take the United Kingdom out of the European Union without a deal but with the opportunity to shift the blame to someone else.”