Boris Johnson has been mocked for saying Theresa May's Brexit strategy would usher in "foreign rule" for the first time since the Battle of Hastings, as he continued his push to kill off her Chequers plan.
The ex-foreign secretary described Ms May's proposals as a "constitutional abomination" and claimed "for the first time since 1066, our leaders were deliberately acquiescing in foreign rule".
Mr Johnson said Irish border proposals would force the UK into becoming a "vassal state" and the talks were on course to end in a "spectacular political car crash", in an article in The Telegraph.
It comes as the prime minister upped her efforts to sell the Chequers blueprint to her party, saying MPs would have to choose between her deal Brexit or no deal at all.
Mr Johnson has thrown his weight behind the rival proposals by the pro-Brexit European Research Group (ERG), led by Jacob Rees-Mogg, where physical checks could be done away from the border, without keeping the UK or Northern Ireland tied to EU customs rules.
He said: "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.
"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."
Labour MP Virendra Sharma, who supports Best for Britain campaign, said Mr Johnson was "Britain's best-know charlatan" and dismissed the comparison to the Norman conquest.
He said: "Chequers is trash - but Johnson neglects to mention how he failed to provide any kind of realistic alternative.
"He would prefer to see chaos on the Irish border than someone else bagging his splash.
"His 1066 reference isn't too far off though. Brexit is like taking an arrow in the eye, and the Brexiteers' constant equivocating won't help. We need a people's vote with the option to stay to sort this out."
In an interview to mark the six-month countdown to Brexit, Ms May told the BBC: "I believe we'll get a good deal, we'll bring that back from the EU negotiations, put that to parliament.
"I think the alternative to that will be not having a deal because a) I don't think the negotiations will have that deal, and b) we're leaving on March 29 2019."
She said the counter-proposal to her Chequers plan is "still a hard border" and her plan was the only way that does not "carve up the United Kingdom".
Ms 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 said.
"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.
"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."
The prime minister said forecasts linked to a no-deal Brexit are "not a prediction", adding: "What is important is how the government responds.
"Under no deal there would be some short-term disruption. It's our job as a government to make sure that we make a success of no deal, just as we make a success of getting a good deal."
Former Lib Dem deputy prime minister Sir Nick Clegg dismissed her claims that there were no other options on the table apart from Chequers or a no-deal Brexit.
"Of all the con tricks we have been subject to in this whole sorry Brexit saga - from the lies in the referendum to the so-called virtues of what is nothing more than an inelegant fudge in Chequers - I think the worst is now to come," he told Today.
"We're now going to be told: 'It's my way or no way' - it's either the Chequers fudge or a sort of cataclysmic cliff edge.
"The idea that the only thing this country should accept is a fudge or the abyss is not only, I think, an insult to the intelligence of British voters, but it's simply not true."
Meanwhile, reports emerged that EU chief negotiator Michel Barnier is drafting a new "protocol" text that includes the use of technology to minimise checks at the Irish border.
Diplomatic papers seen by The Times said: "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."
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