* PM Johnson says backstop must be abolished

* Johnson Vows to make UK “greatest place on earth”

* UK plots Brexit gambit to get a deal

* EU digs in heels, says existing deal is “best and only” one (Adds call between Johnson and Juncker, updates sterling)

By Guy Faulconbridge and Kylie MacLellan

Johnson, who was hailed by the U.S. president as Britain’s Trump, has promised to strike a new divorce deal with the European Union and to energise the world’s fifth-largest economy after what he casts as the gloom of Theresa May’s premiership.

On entering Downing Street on Wednesday, Johnson set up a showdown with the EU by vowing to negotiate a new deal and threatening that, if the bloc refused, he would take Britain out on Oct. 31 without a deal to limit economic dislocation.

“Our mission is to deliver Brexit on the 31st of October for the purpose of uniting and re-energising our great United Kingdom and making this country the greatest place on earth,” Johnson proclaimed.

He said Britain could be the most prosperous economy in Europe by 2050, a feat that would mean drawing far ahead of France and then overtaking Germany.

Johnson promised that British children and grandchildren “will be living longer, happier, healthier, wealthier lives”.

STERLING FALLS AGAIN

Johnson’s ascent has placed an avowed Brexiteer in charge of the British government for the first time since the 2016 EU Brexit referendum, which shocked the world and upset financial markets.

Sterling has lost more than 5% of its value since early May and recently touched a 27-month low against the dollar. It fell further on Johnson’s first day in office, trading at around $1.2450.

Trump has repeatedly praised Brexit and has advised the United Kingdom to “walk away” if the EU offers a poor deal. While he grew frustrated with May, Trump said this week he liked her successor.

Johnson spiced his pitch to the EU on Thursday by bluntly stating that one of the most hotly contested elements of the Brexit divorce agreement would have to be struck out if there was to be an orderly exit.

His bet is that the threat of the economic disruption that a “no-deal” Brexit would cause will convince the EU’s biggest powers – Germany and France – to agree to revise the divorce deal that May agreed last November but failed to get ratified.

Johnson told parliament he would not accept the Irish backstop, an insurance policy designed to prevent the return of a hard border between the Irish Republic and Northern Ireland by provisionally keeping Britain in a customs union with the EU:

“It must be clearly understood that the way to the deal goes by way of the abolition of the backstop.”

EU Brexit negotiator Michel Barnier quickly rejected that condition.

“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,” Barnier said in a note sent to EU member states.

European Commission President Jean-Claude Juncker also warned Johnson in a phone call that the deal the EU struck with May last year was “the best and only” one, an EU spokeswoman said.

Johnson stuck to his guns, telling Juncker that the backstop had to go, a Downing Street spokesman said.

Both sides agreed to stay in contact.

NO MAJORITY

The Irish backstop is contained in a protocol of the Withdrawal Agreement that Johnson’s predecessor, Theresa May, agreed to in November.

It is the most contentious part of the deal for British lawmakers, who fear it could ultimately slice Northern Ireland off from the rest of the United Kingdom.

Johnson’s government does not have a majority in parliament, and so rules with the help of 10 Northern Irish lawmakers from the Democratic Unionist Party, who vehemently oppose the backstop.

Irish Prime Minister Leo Varadkar on Wednesday that Johnson’s pledge to secure a new Brexit deal was “not in the real world”.

While repeatedly refusing to countenance rewriting the Withdrawal Agreement, the EU has said it could change the non-binding “Political Declaration” on future ties that is part of the divorce deal.

If EU leaders refuse to play ball and Johnson moves towards a no-deal Brexit, some of the lawmakers in his own Conservative party have threatened to thwart what they see as a blind leap into economic chaos.

Faced with that prospect, Johnson could try to call an election in a bid to override them.

(Reporting by Guy Faulconbridge; additional reporting by Gabriela Baczynska in Brussels; Editing by Jon Boyle and Kevin Liffey)