Boris Johnson says he wants the United Kingdom to become Europe's largest economy by 2050, in a bid to make Britain the "greatest place on Earth", during his maiden speech to Parliament as Prime Minister.

Key points: A backstop keeps Northern Ireland in a customs union with the EU to avoid a border

A backstop keeps Northern Ireland in a customs union with the EU to avoid a border It was designed to maintain EU border controls and minimise cross-border violence

It was designed to maintain EU border controls and minimise cross-border violence The new Cabinet is stacked with ardent Brexiteers, flushing the old guard out

Mr Johnson — whom United States President Donald Trump praised as a British version of himself — 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.

Upon entering Downing Street on Wednesday, Mr Johnson set up his Government for a showdown with the EU by vowing to negotiate a new divorce deal and threatening to leave the bloc without a deal if Brussels refused.

"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," Mr Johnson told the House of Commons.

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

In 2018, the United Kingdom sat behind Germany's Gross Domestic Product by $1.69 trillion.

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

Boris begins by binning backstop

Northern Ireland voted to remain with the European Union by 56 per cent during the 2016 Brexit referendum. ( ABC News: James Glenday )

The Prime Minister told MPs that the backstop — an insurance policy designed to prevent the return of a hard border between the Irish Republic and the United Kingdom's 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," Mr Johnson said.

The Irish backstop is contained in a protocol of the Withdrawal Agreement that Mrs May proposed in November.

It is the most contentious part of the deal for British MPs, as Northern Ireland will be kept in a customs union with the EU in lieu of a formal border on the island of Ireland.

This is an anathema to ardent unionists, including Northern Ireland's Democratic Unionist Party — whose 10 MPs hold the balance of power in the Commons — who perceive it as a threat to UK unity.

When asked about Mr Johnson's comment, Irish Prime Minister Leo Varadkar said he looked forward to discussing the issue with the new PM.

Mr Varadkar told reporters that Mr Johnson's no-deal Brexit was a fiction. ( Reuters: Clodagh Kilcoyne )

Mr Varadkar yesterday said Mr Johnson's pledge of a new Brexit deal was "not in the real world".

Mr Johnson's dramatic rise to the UK's top job sets the world's fifth-largest economy up for a showdown with the EU and a potential constitutional crisis, or election, at home, as MPs have vowed to thwart a no-deal Brexit.

He has promised to do a new Brexit deal with the bloc in less than 99 days but has warned that if EU leaders refused — something he said was a "remote possibility" — then Britain would leave without a deal, "no ifs or buts".

Mr Johnson's position has prompted former Conservative British Prime Minister John Major to vow to "save the Queen" from a constitutional crisis if he suspended Parliament to get a no-deal through.

The new Prime Minister's bet is that the threat of a no-deal Brexit will persuade the EU's biggest powers — Germany and France — to agree to revise Mrs May's divorce deal, which failed to get through the Commons thrice.

Brussels is not for turning

Outgoing EU Commission President Jean Claude-Juncker (right) has ruled out renegotiation. ( AP: Virginia Mayo )

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," Mr Barnier said in a note sent to EU member states.

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

In those circumstances, he could call an election in a bid to override them.

Mr Johnson began his time in office by decisively sweeping away Mrs May's cabinet in one of the biggest culls of senior government jobs in recent British history — dubbed a "summer's day massacre" by one MP.

Earlier on Thursday the Prime Minister held his first full meeting of the Cabinet, in which Brexiteers now dominate the senior posts.

Mr Johnson held his first Cabinet meeting at Downing Street on Thursday. ( AP: Aaron Chown, pool )

"Night of the Blond Knives", read The Sun's headline, Britain's most-read newspaper, a reference to Mr Johnson's hair and the Nazi's killings of a swathe of German politicians on one night in 1934.

A total of 17 ministers in Mrs May's government either resigned or were sacked, creating a powerful new group of enemies in Parliament.

Most of Mr Johnson's senior appointees are Brexit supporters.

Sajid Javid, 49, was named as his Finance Minister. He is a Eurosceptic who voted to remain in the 2016 referendum.

Others are avowed Brexiteers: Priti Patel was appointed Interior Minister, Dominic Raab was appointed Foreign Minister and Stephen Barclay remained as Brexit Minister.

Mr Johnson also appointed Dominic Cummings, the campaign director of the official Brexit Vote Leave campaign, as a senior adviser in Downing Street.

ABC/Reuters