The prime minister will call on her MPs to ignore claims from Mr Johnson that her plans are “a cheat” and instead ask them to put the “national interest” first by backing them to avoid a catastrophic no-deal Brexit.
The row between the pair boiled over on Tuesday with the ex-foreign secretary claiming Ms May’s Chequers proposals would leave the Britain “in manacles”, while a “cross” prime minister argued her rival’s plans would destroy the UK.
Ms May will also take a series of veiled swipes at her leadership rival in her keynote speech on the final day of Tory conference – telling delegates the British people wish their party to remain “moderate”.
But further pressure was also heaped on Ms May’s Brexit strategy from the DUP, her Northern Irish partners in government, who threatened to pull their support over a compromise with Brussels she is said to be planning.
There were also warnings from Europe that politicians in Brussels are not willing to offer Ms May a deal that would cross their red lines simply to save her political skin
With Tory rebels wrestling over whether to vote down Ms May’s Brexit plans due to their ideological dislike of the EU, officials said she would remind them Conservatives “always act in the national interest and put the needs of hardworking people first”.
Her ministers had spent the week warning of the terrible consequences of a no-deal Brexit for the economy and ordinary people, if a deal cannot be reached.
But on Tuesday Mr Johnson told an auditorium packed full of Conservatives that her plans would “leave the UK effectively paraded in manacles down the Rue de la Loi”.
Ms May hit back in a television interview in which she said his idea of a Canada-style trade deal would mean an unacceptable border in the Irish Sea.
Asked about his speech, Ms May said she had not watched it, because she had been “meeting activists” and “seeing a party that’s in really good heart”.
But she told the BBC: “There are one or two things that Boris said that I’m cross about.
“He wants to tear up our guarantee to the people of Northern Ireland [that it will remain under the UK’s jurisdiction].”
In her speech on Wednesday she will continue her criticism of her rival, saying that the public want to back a party that is, “decent, moderate and patriotic, one that puts the national interest first, delivers on the issues they care about – and is comfortable with modern Britain in all its diversity.
“We must show everyone in this country that we are that party.”
Her words come after weeks in which Mr Johnson has used increasingly extreme and offensive language in articles, including referring to Muslim women in burqas and niqabs as looking like “bank robbers” and “letter boxes” and then saying Ms May’s plans would put the UK in a “suicide vest”.
At a fringe event the DUP’s Westminster leader Nigel Dodds commented on claims that even Ms May’s proposals for a Brexit deal could see some “regulatory” checks undertaken at Irish Sea ports.
Threatening to pull support for her approach, he said: “Let us be clear that for the DUP there will be no border of any kind down the Irish Sea – customs, regulatory, political, constitutional or otherwise because we are part of the UK and we will leave the EU together and as one nation.
“The danger of [the EU’s proposed] Irish backstop is that it has the potential, to not only separate Northern Ireland from the rest of the UK ... it also has the potential to shackle the UK for generations to come in its relationship with Europe.”
Ms May will attempt to convince her cabinet that proposed compromises on customs and future EU regulation are not a step too far and will secure a deal.
But in Brussels the European Parliament’s Brexit chief Guy Verhofstadt warned Ms May the EU will not abandon its red lines to “rescue” her party from a looming disaster of its own making.
“We will never undermine the principles of the European project to rescue a political party who is not even capable to find a common line on the mess of Brexit,” he told MEPs.
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