LONDON — Britain's new Prime Minister Boris Johnson urged the EU on Thursday to "rethink" their opposition to renegotiating the current Brexit deal — only to be immediately rebuffed by Brussels.

In a pugnacious parliamentary debut as leader, the former London mayor warned that the terms of the agreement struck by his predecessor Theresa May were "unacceptable" and would "sign away our economic independence".

"I hope that the EU... will rethink their current refusal to make any changes to the withdrawal agreement," Johnson told MPs.

"If they do not, we will of course have to leave the EU without an agreement," he added, vowing to "turbo-charge" preparations ahead of the latest deadline of Oct. 31.

The former foreign secretary also threatened to withhold the £39 billion ($49 billion) divorce bill that Britain has previously said it owes the European Union and spend it instead on preparing for a no-deal outcome.

Just hours later, the EU's chief Brexit negotiator Michel Barnier warned that Johnson's demands were "unacceptable", in an email to member state ambassadors seen by AFP that described his speech as "rather combative".

"PM Johnson has stated that if an agreement is to be reached it goes by way of eliminating the backstop," Barnier wrote, referring to a contentious element of the current deal to keep the Irish border open under all scenarios.

"This is of course unacceptable and not within the mandate of the European Council." With Britain's position appearing to harden, Johnson will talk by telephone with the president of the European Commission Jean-Claude Juncker later on Thursday.

Johnson addressed a raucous session of parliament, in which he was repeatedly shouted down by opposition MPs, a day after initiating a radical overhaul of the Cabinet. Purging more than half the ministers in May's team, he installed what some have described as the most right-wing British government in decades.

Johnson argues that his threat of a chaotic end to Britain's EU involvement will force Brussels to relent and give London better terms that would let it pursue trade deals with powers such as China and the United States.

Brexit backers in parliament had accused May of ignoring voters' wishes by promising to keep the UK tied to the bloc's economic rules if necessary to preserve a free-flowing border between EU member Ireland and Britain's Northern Ireland.

Johnson's solution for the frontier revolves around proposals that have been rejected as either unworkable or insufficient by both EU and Irish leaders. Irish Prime Minister Leo Varadkar — his heavily trade-dependent nation standing to lose most from a messy EU-UK split — reiterated Thursday his call for compromise. "I hope that the new UK prime minister hasn't chosen no deal," he said. — AFP