Johnson, who is expected to defeat Foreign Secretary Jeremy Hunt in the election to become the next Conservative Party leader and British prime minister, has said Britain can rely on a provision in global trade rules to make sure trade relations remain unchanged.
Junker said: "We repeated unanimously that there will be no renegotiating of the withdrawal agreement".
Donald Tusk has also accused the United Kingdom of "wasting time" over Brexit as the Tories plunge into a month-long leadership battle.
European Commission President Jean-Claude Juncker pauses before speaking during a media conference at the end of an EU summit in Brussels, Friday, June 21, 2019.
It is expected to be Ms May's last summit as leader.
Britain was originally set to leave the European Union on March 29, but because of the British Parliament's failure to back the deal that Ms.
He joked that the next round of Brexit might be "more exciting" due to personal feelings in London, a reference to the inevitability of a "hard-Brexit" candidate in 10 Downing Street.
Mark Carney, the head of the Bank of England, on Friday dismissed suggestions from Mr. Johnson that tariffs on trade with the European Union can be avoided even if the country leaves the bloc without a withdrawal agreement.
"Gatt applies if you have an agreement, not if you have decided not to have an agreement or have been unable to come to an agreement".
Carney indicated there's only so much firms can do to offset the impact of tariffs.
He has said he would be prepared to delay Brexit if an agreement was in sight, but would walk away if there was no sign of movement from the EU.
Fervent Brexit campaigner Boris Johnson or foreign minister Jeremy Hunt will take over from Prime Minister Theresa May in late July, with the flamboyant Johnson the odds-on-favorite to win.
"A damaging no-deal Brexit would cause short-term disruption to our economy, soaking-up all the fiscal headroom we have built", Hammond told delegates. Brexit has been delayed twice already and both Hunt and Johnson say Britain should leave the bloc on the current deadline date of October 31 - even if it means walking away with no deal.