The United Kingdom‘s formal proposals are due to be submitted to Brussels later this week, and Johnson said on Tuesday it will soon become apparent if there is “no way of getting it over the line from their point of view”.
Johnson has urged leaders in Brussels, Dublin and Berlin to work with him as the “rubber hits the road” in efforts to strike a deal before the scheduled Brexit date on October 31.
His comments came after Dublin rejected reported proposals for customs posts along both sides of the Irish border to replace the backstop.
Irish state broadcaster RTE reported that the suggestion sent to the EU by the UK would lead to the posts being built between 3km and 6km (five and 10 miles) back from the current border.
Simon Coveney, deputy Irish premier, said Northern Ireland and the Republic of Ireland “deserve better”.
But Johnson said those were preliminary ideas that had been floated rather than the formal proposals which are expected to be set out later this week after the conference of his Conservative Party finishes on Wednesday.
“They are not talking about the proposals we are going to be tabling, they are talking about stuff that went in previously,” he told the BBC.
“But clearly this is the moment when the rubber hits the road. This is when the hard yards really are in the course of the negotiations.”
The Irish border question has become the main stumbling block to a Brexit deal.
The backstop – a contingency plan which would keep Northern Ireland closely aligned to Brussels’ customs and regulatory rules if no other method is found to prevent a hard border – is loathed by Brexiteers and Johnson is determined to remove it from the Withdrawal Agreement that predecessor Theresa May negotiated with the EU.
“The difficulty really is going to be around the customs union and to what extent Northern Ireland can be retained within EU bodies at all,” Johnson said.
“We’re going to make a very good offer, we are going to be tabling it very soon, but there is a difficulty if you try to keep Northern Ireland in a customs union because one of the basic things about being a country is you have a single customs perimeter and a single customs union.”
The idea for the customs posts was contained in the so-called “non-papers” submitted by UK officials during recent technical discussions.
European Commission spokeswoman Mina Andreeva said: “We have not received any proposals from the United Kingdom that meet all the objectives of the backstop, as we have been reiterating and demanding.”
She added: “It’s the UK’s responsibility to come forward with workable and legally operational solutions that meet all of the objectives of the backstop.”
Shadow Brexit secretary Sir Keir Starmer said: “If Boris Johnson had spent any time listening to businesses and communities in Northern Ireland, he would know that these proposals are utterly unworkable.”
But Johnson insisted “that is not what we are proposing at all”, telling BBC Radio 4’s Today programme it was “absolutely not” true that he wanted to create a hard border a few miles away from the actual border.
But he said it was a “reality” that some checks would be needed to create a “single customs territory” for the UK once it leaves the EU.
Meanwhile, The Times reported that Johnson’s plan to get around the Benn Act – the law aimed at preventing a no-deal Brexit without MPs’ approval – would be to ask EU leaders to rule out any extension to the October 31 deadline.
Johnson denied that was the case, adding: “In truth, we have not made any such request.”
Talks between Labour, the Liberal Democrats, the SNP and other opposition forces broke down on Monday over the crucial issue of who should be installed as a caretaker prime minister.
Labour’s Jeremy Corbyn, as the leader of the principal opposition, would be the obvious first choice candidate to lead a “unity government” – but Liberal Democrats leader Jo Swinson quashed the idea.
“Jeremy Corbyn is not going into Number 10 on the basis of Liberal Democrats’ votes,” she said.
However, Ian Blackford, the Scottish Nationalist Party’s Westminster leader, urged opposition parties to “go the extra mile” and act together to remove Johnson, who he described as “toxic”.
“This is not about putting someone in as prime minister,” he said. “It is a means to an end.”
“All of us have got to recognise the responsibility we have, it is up to the others that have failed to do that to recognise the challenge, and woe betide anyone that puts us in the situation that we run that risk of Boris Johnson driving us out on a no-deal basis – they will pay a price for that.”
On Tuesday, John McDonnell, Labour’s deputy leader, admitted a vote of no confidence in Johnson’s administration would not be likely until after a crunch EU summit this month.