LONDON (REUTERS) - Boris Johnson promised on Thursday (July 25) that Brexit would make Britain the greatest place on earth, echoing the patriotic rhetoric of US President Donald Trump in a debut speech as prime minister before Parliament.

Johnson, who was hailed by the US president as Britain's Trump, has promised to strike a new Brexit 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 the United Kingdom up for a showdown with the EU by vowing to negotiate a new divorce deal and threatening that if the bloc refused then he would leave without a deal on Oct 31.

"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 told parliament in his first speech as prime minister.

He said he was not being hyperbolic as the United Kingdom could be most prosperous economy in Europe by 2050, a feat that would mean drawing far ahead of France and then overtaking Germany.

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

STERLING STEADY

Johnson's victory has placed an avowed Brexiteer in charge of the British government for the first time since the 2016 EU referendum which shocked the world and roiled financial markets.

Sterling, which has lost more than 5 per cent of its value since early May and recently touched a 27-month low against the dollar and a six-month low versus the euro, was little changed on Johnson's first day in office, trading below US$1.25.

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 former prime minister Theresa May, Trump said this week he liked Johnson.

Johnson spiced his pitch to the EU on Thursday by bluntly stating that one of the most controversial 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 a no-deal Brexit will persuade 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 the Irish backstop, an insurance policy designed to prevent the return of a hard border between the Irish Republic and Northern Ireland, must be abolished.

"It must be clearly understood that the way to the deal goes by way of the abolition of the backstop," Johnson said in his first speech as prime minister.

The EU's top Brexit negotiator Michel Barnier quickly rejected Johnson's demand as unacceptable.

"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.

NO MAJORITY

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

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

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

When asked about Johnson's comment, Irish Prime Minister Leo Varadkar said he looked forward to discussing the issue with Johnson. Varadkar yesterday said Johnson's pledge of a new Brexit deal was "not in the real world".

The EU has so far repeatedly refused to countenance rewriting the Withdrawal Agreement but has said it could change the "Political Declaration" on future ties that is part of the divorce deal.

Johnson was to speak by telephone with the European Union's chief executive Jean-Claude Juncker later on Thursday, an EU spokeswoman said.

If EU leaders refuse to play ball with Johnson and he moves towards a no-deal Brexit, some British lawmakers have threatened to thwart what they cast as a disastrous leap into economic chaos.

In those circumstances, Johnson could call an election in a bid to override lawmakers.