Rejecting a Brexit agreement based on the government's Chequers plan will mean the UK crashed out of the EU without any deal at all, Theresa May has said in a warning to Brexiteers in her own party.
The Prime Minister said that unless Conservative MPs accept the outcome of her negotiations with Brussels, "the alternative to that will be having no deal".
Her comments in a BBC Panorama interview came as Boris Johnson mounted another attack on Mrs May's strategy, claiming that the issue of the Irish border is being used to turn the UK into a 'vassal state' and that talks will end in a "spectacular political car crash".
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Mr Johnson said the European Union's fallback position for the Irish border would mean Northern Ireland was "annexed" by Brussels.
Alternative plans set out by Mrs May would "effectively" keep Britain in the bloc, he added.
However the PM has said the counter-proposal to her Chequers plan is "still a hard border" and hers is the only way that does not "carve up the United Kingdom" and put the Union at risk.
Reports suggest that the EU is preparing to accept use of technology to avoid the need for new border infrastructure in Ireland and checks on goods being shipped between Northern Ireland and the rest of the UK.
Brussels' current proposal is to effectively keep Northern Ireland in the EU customs union until a comprehensive trade deal with the UK is agreed, something that the DUP and political parties in Scotland have rejected.
Mr Johnson has backed proposals by the pro-Brexit European Research Group that physical checks can be done away from the border, without keeping the UK or Northern Ireland tied to EU customs rules.
Writing in The Daily Telegraph, he said: "If the Brexit negotiations continue on this path they will end, I am afraid, in a spectacular political car crash."
"If we are to get out of this mess, and get the great British motor back on track, then we need to understand the Irish backstop, and how it is being used to coerce the UK into becoming a vassal state of Brussels," he added.
"The EU's backstop would leave a border down the Irish sea while the UK's proposal left it "volunteering" to "remain effectively in the customs union and large parts of the single market until Brussels says otherwise", Mr Johnson said.
"Both versions of the backstop are disastrous," he wrote. "One threatens the union; the other version - and its close cousin, Chequers - keep us effectively in the EU, as humiliated rules takers.
"We need to challenge the assumptions of both these Irish backstops, or we are heading full throttle for the ditch with a total write-off of Brexit.
"We are straining at the gnat of the Irish border problem - in fact we haven't even tried to chew the gnat - and we are swallowing the camel of EU membership in all but name."
Mrs May said there needs to be "friction-free" movement of goods across the Irish border, without customs or regulatory checks between the UK and EU, after Brexit.
She said that the counter-proposal will not "solve the issue of no hard border by having a hard border 20km inside Ireland".
"The people of Northern Ireland deserve to be listened to in these negotiations by the UK Government, as do people elsewhere in this country," she told the BBC.
"I want to ensure that as we go forward we have that strong union... Northern Ireland is part of the United Kingdom. They don't want a hard border between Northern Ireland and Ireland.
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"The only proposal that's been put forward that delivers on them not having a hard border and ensures that we don't carve up the United Kingdom, is the Chequers plan."
EU chief negotiator Michel Barnier is drafting a new "protocol" text that includes the use of technology to minimise checks at the Irish border, according to The Times.
Diplomatic notes seen by the paper state: "The biggest unsolved problem is Northern Ireland. There is a political mobilisation in the UK in this regard. Therefore, we are trying to clarify the EU position."