Michel Barnier has called Boris Johnson’s Brexit approach “rather combative” and “unacceptable”.
The European Union’s chief Brexit negotiator told EU member states they must prepare for the new Prime Minister to prioritise planning for a no deal withdrawal.
“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,” Mr 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.”
He also noted the “many strong reactions” to Mr Johnson’s debut House of Commons speech as leader, and said he will carefully follow the domestic response.
The memo came shortly before Mr Johnson was due to speak on the phone with outgoing European Commission president Jean-Claude Juncker.
In his first Prime Minister’s Questions he said he was confident he would be able to negotiate a fresh deal by October, and ruled out putting forward his predecessor’s agreement.
“Its terms are unacceptable to this Parliament and this country,” he said.
Downing Street would not respond directly to Mr Barnier’s comments, however the Prime Minister’s spokesman said to wait and see.
“It's day one, the PM has said he wants a deal and is going to be energetic in the pursuit of that,” he said.
“The fact remains, however, that the Withdrawal Agreement has been rejected three times by this Parliament and is clearly not acceptable to the current UK Parliament.”