European Union's chief negotiator Michel Barnier. Photograph:( AFP )
Johnson has threatened to withhold the $49 billion divorce bill that Britain has previously said it owes the European Union.
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 October 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.
A Downing Street spokesman declined to comment on Barnier's comments, adding: "It's day one."
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.
The 55-year-old has assembled a team of social conservatives and Brexit hardliners who argue that leaving the EU after 46 years without an agreement will be less painful than economists warn.
The markets were relieved by the appointment of former Deutsche Bank Sajid Javid as finance chief.
The pound held steady against the dollar and euro as traders waited for Johnson's first policy moves.
Other appointments were more divisive.
Brexit hardliner Dominic Raab became foreign secretary and Jacob Rees-Mogg -- leader of a right-wing faction of Conservatives who helped bring about May's demise -- as the government's parliament representative.
New interior minister Priti Patel has previously expressed support for the death penalty and voted against same-sex marriage.
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.
Johnson will have the backing of his governing Conservative party but not the nation in his first days in office.
He beat the now-former foreign secretary Jeremy Hunt by a two-to-one margin in a vote held by fewer than 160,000 paying members of the Conservatives.
But a YouGov survey found his approval rating in Britain as a whole at just 31 per cent. Johnson is also immediately confronted by issues beyond Brexit.
Iran's seizure last Friday of a British-flagged tanker in the Gulf thrusts him into the middle of the Islamic republic's escalating standoff with US President Donald Trump.
Johnson boasts a friendship with Trump that his doubters criticise but supporters say could boost Britain's chances of clinching a post-Brexit trade deal with the United States.