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The EU's chief Brexit negotiator Michel Barnier has again rejected Boris Johnson's demands for the Irish backstop is scrapped.

Mr Barnier insists that the controversial insurance plan for the Irish border in the Withdrawal Agreement represents the "maximum flexibility" that Brussels can offer.

The Prime Minister has called on EU leaders to discard the so-called backstop and branded it as "unacceptable", warning the arrangement must be ditched if a no-deal Brexit is to be avoided.

MPs voted against the Withdrawal Agreement, negotiated by his predecessor Theresa May, three times.

Writing in the Sunday Telegraph, Mr Barnier said the backstop had to stay to protect the integrity of the EU's single market while ensuring an open border on the island of Ireland.

He said: "After two years of painstaking talks, the EU and the UK reached an agreement on operational solutions for a whole range of areas where the UK's withdrawal creates uncertainty.

“The backstop is the maximum amount of flexibility that the EU can offer to a non-member state,” he added.

Mr Barnier also hit back against Mr Johnson's claims that a no-deal Brexit would be the fault of the EU.

“I am not optimistic about avoiding a no-deal scenario, but we should all continue to work with determination,” Mr Barnier said.

"On the EU side, we had intense discussions with EU member states on the need to guarantee the integrity of the EU's single market, while keeping that border fully open.

"In this sense, the backstop is the maximum amount of flexibility that the EU can offer to a non-member state."

The Irish backstop is a contingency measure negotiated by Mr Johnson's predecessor Theresa May to get an exit deal over the line which could see Northern Ireland aligning with some EU single market rules if a trade deal is not agreed by the UK and EU after Brexit.

Meanwhile, Mr Johnson, who has vowed to take Britain out of the EU with or without a deal on October 31, restated his case for a harder Brexit. saying the country needed to "come out of the incarceration of the backstop".

He told the Sunday Times: "Everybody understands what is wrong with the current withdrawal agreement: it keeps the UK locked into the EU. It means they can boss us around on trade policy or on how we legislate forever."

His de facto deputy Michael Gove also said that to remove the option of a no-deal Brexit on Oct 31 would “diminish” the “chances of securing changes” to the Brexit deal that could get it passed through parliament.

Ratcheting up the rhetoric, Mr Johnson's also warned that Tory MPs faced a "choice" between him and Jeremy Corbyn , ahead an expected clash this week when no-deal opponents will are expected to try to seize control of the parliamentary agenda to push through legislation delaying Brexit beyond October 31.

He told the Sunday Times: "I just say to everybody in the country, including everyone in parliament, the fundamental choice is this: are you going to side with Jeremy Corbyn and those who want to cancel the referendum?

"Are you going to side with those who want to scrub the democratic verdict of the people - and plunge this country into chaos?"