Boris Johnson’s Brexit deal offer is a “scam” designed to push Britain into a no-deal exit from the EU, Tony Blair’s former chief of staff has said.

Jonathan Powell, one of the architects of the Good Friday Agreement that helped bring peace to Northern Ireland, said the prime minister’s proposals were “nonsense” and a “pretense” that suggest he is not aiming for a deal.

British officials have made clear that the legal texts which will be presented to the EU on Wednesday are a final offer, and unless Brussels is prepared to engage there will be no more talks until after the October 31 Brexit.

The Daily Telegraph reported that Johnson will use his Tory conference closing speech to unveil a “two borders for four years” plan on Wednesday that will leave Northern Ireland in a relationship with Europe until 2025.

“Major EU capitals” have already been briefed about the plan, which accepts the need for both a regulatory border between Britain and Northern Ireland in the Irish Sea for four years and customs checks between the North and the Irish Republic, the paper reports.

Powell said it would impose a hard border on the island of Ireland, “just the opposite of what we promised at the beginning of this negotiation with the EU”.

“It’s not therefore going to be acceptable to the EU, it’s not going to be acceptable to Northern Ireland to the majority of the population and it is not going to be acceptable in the Republic of Ireland,” he told BBC Newsnight.

“It is not a serious proposal, and presumably what they are doing is trying to avoid a deal to get to no deal, as we always assumed they were going to do.

“This is the final confirmation that that’s their aim.”

“It looks to me like something that has been put forward by someone who is not serious (about reaching a deal), that actually intends to leave without a deal.”

Ireland’s deputy prime minister, Simon Coveney, speaking late on Tuesday night, said this was “no basis for an agreement” and “concerning to say the least”.

“I read that Prime Minister Johnson is going to bring forward a proposal tomorrow. Some are even saying he has been briefing certain EU capitals in relation to these ideas since Tuesday. We haven’t seen anything,” Coveney said on Virgin One’s Tonight Show.

Powell said the customs checks envisaged in the plan would undermine the Good Friday Agreement.

“The point of this is not how long it takes a lorry to cross the border in Northern Ireland, that is not the issue,” he said.

“The issue is identity. The Good Friday Agreement tried to solve the issue of identity by allowing people in Northern Ireland to live there and feel Irish or feel British or feel both.

“If you put in a customs border, as you have to if you leave the customs union, then that will be destroyed.”