Boris Johnson's Brexit demands are "unacceptable," according to the EU's chief negotiator.

The new UK prime minister has demanded the complete removal of the backstop from the Brexit deal.

If not, the UK would be prepared to walk away without a deal in October, Johnson said Thursday.

However, chief negotiator Michel Barnier wrote to EU leaders describing the demand as "unacceptable" and urging them to stay united in response to Johnson's "combative" rhetoric.

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The European Union's chief Brexit negotiator has described Boris Johnson's Brexit demands as "unacceptable" and urged EU leaders to remain united in response to the new prime minister's "combative" approach.

In a letter to European leaders leaked to Politico, Michel Barnier flatly rejected Johnson's demand for the contentious backstop for Northern Ireland to be removed from the Brexit deal altogether.

"PM Johnson has stated that if an agreement is to be reached it goes by way of eliminating the backstop," Barnier said.

"This is of course unacceptable and not within the mandate of the European Council."

Prime Minister Johnson on Thursday told Members of Parliament that he would not accept compromises on the backstop — including a fixed time-limit — and that only its total "abolition" would be acceptable.

The backstop is the fallback measure for preserving the status quo on the island of Ireland and ensuring there is no hard border between the Republic of Ireland and the United Kingdom.

The inclusion of the backstop in the Withdrawal Agreement is partly why so many Conservative MPs voted it down three times. If activated, it would put the UK in a customs union with the EU for an indefinite length of time.

However, Barnier in his private letter repeated the EU's public position that removing the backstop was not an option, and that the EU side would only negotiate with Johnson on terms it deems acceptable.

"While he has declared that he will only engage with the EU on this basis, we are on our side ready to work constructively, within our own mandate," the EU's chief Brexit negotiator said.

"We will analyze any UK idea on withdrawal issues that are compatible with the existing WA [Withdrawal Agreement], and we are of course ready to rework the political declaration, in line with the EUCO [European Council] guidelines."

EU ready to face 'combative' Johnson

Barnier also urged EU leaders to "remain clam, stick to our principles and guidelines and show solidarity and unity" in response to Prime Minister Johnson's "combative" first full day in power on Thursday.

In his first House of Commons statement, Johnson doubled down on his threat to leave the EU without a deal on October 31 if the backstop isn't removed, and described the current deal as "divisive" and "anti-democratic."

"No country that values its independence, and indeed its self-respect, could agree to a treaty which signed away our economic independence and self-government as this backstop does," Johnson told MPs.

Michel Barnier. Charles McQuillan/Getty

In his letter to European leaders, Barnier wrote: "As suggested by his rather combative speech, we have to be ready for a situation where he gives priority to the planning for 'no deal,' partly to heap pressure on the unity of the EU27.

"No deal will never be the EU's choice, but we all have to be ready for all scenarios."

Naomi Smith, CEO of anti-Brexit group Best For Britain, said: "Johnson doesn't want what's best for Britain, but for himself. He's more concerned with shoring up support from Brexit zealots, than protecting jobs, rights, and our keeping our country together.

She added: "Barnier is right not to give in to bluff and bluster, but to do what's right for the British people, who remain EU citizens."