The European Union's chief Brexit negotiator warned member states on Thursday that Prime Minister Boris Johnson's demands to change Britain's withdrawal agreement were unacceptable.

"PM Johnson has stated that if an agreement is to be reached it goes by way of eliminating the backstop. This is of course unacceptable and not within the mandate of the European Council," Barnier wrote, in an e-mail to EU ambassadors seen by AFP.

Earlier, in his first appearance before the House of Commons as prime minister, Johnson had used the same word, "unacceptable", to describe the Brexit withdrawal agreement signed by his predecessor.

In particular, he denounced the "backstop", a mechanism in the agreement to keep Britain in the EU customs union until a future trade agreement is reached to keep the Irish border open.

Barnier, in his message to the other 27 member states, noted the "combative" tone of Johnson's speech, but noted that he has no mandate from the 27 EU leaders to renegotiate the deal.

And he warned that Johnson's decision to focus Britain's efforts on planning for a "no deal" Brexit on October 31 could be an attempt "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," Barnier wrote.

Barnier also urged EU members to keep an eye on domestic British opposition ot Johnson's hardline position

I note also the many strong reactions to the speech in the House of Commons. In this context we must follow carefully the further political and economic reactions and developments in the UK following this speech," he wrote. 

"In any case, what remains essential on our side is to remain calm, stick to our principles and guidelines and show solidarity and unity of the 27."

Johnson was due to have a phone call with EU Commission president Jean-Claude Juncker later Thursday.