Johnson, the face of the "Leave" campaign in Britain's 2016, EU membership referendum, will become prime minister on Wednesday, succeeding Theresa May, after he campaigned for the leadership of his Conservative Party promising to deliver Brexit on Oct. 31, with or without a divorce deal with the EU.

The bloc has congratulated Johnson on his victory but was firm that it would not offer Britain better departure terms.

The message was reiterated on Wednesday by a group of EU lawmakers dealing with Brexit.

"The (group) notes that recent statements, not least those made during the Conservative Party leadership campaign, have greatly increased the risk of a disorderly exit of the UK," it said in a statement.

"A no-deal exit would be economically very damaging, even if such damage would not be inflicted equally on both parties."

The European Parliament must endorse any EU-UK Brexit deal and has said the bloc is ready to amend a declaration on their future ties, which accompanies the legally-binding divorce agreement.

Johnson wants changes to the divorce deal, notably removing the so-called Irish backstop, an insurance policy that could tie the UK to some of the EU's trading rules after Brexit to avoid deploying extensive checks on the Irish border.

Brexit backers fear that would make it hard for Britain to strike independent trade deals around the world.

"An orderly exit is only possible if citizens' rights, the financial settlement and the backstop, that in all circumstances ensures no hardening of the border on the Island of Ireland... are guaranteed," the European Parliament's Brexit group said.

"The Withdrawal Agreement ... cannot be reopened."

May agreed to the deal with the EU last November but the British parliament has since voted it down three times.

The bloc is preparing for a no-deal exit or another delay to Britain's departure date.