Michel Barnier, the EU's chief Brexit negotiator, has slammed Boris Johnson's desire to renegotiate the UK's divorce deal with the bloc.
Johnson became the UK's new prime minister on Wednesday on a ticket of leaving the EU with or without a deal by October 31.
He has said the Irish backstop part of the agreement negotiated by his predecessor, Theresa May, is dead and will have to be removed.
But, in comments that emerged on Thursday afternoon, Barnier said the stance from the UK's new prime minister is "unacceptable".
Barnier sent an email to EU member states, warning that Johnson's "combative" speech was indicative of a no-deal outcome. He said it was essential that the EU "stick to our principles and guidelines and show solidarity and unity of the 27".
The backstop is essentially an insurance policy to avoid a hard border — such as border posts — between Northern Ireland (part of the UK) and the Republic of Ireland (part of the EU), post-Brexit.
Hardline Brexiters fear that if they sign up to this it could keep the UK in the EU indefinitely.