UK Prime Minister Boris Johnson met his Brexiteer-dominated team of senior ministers on Thursday and said his new government was fully committed to the "momentous task" of leading Britain out of the European Union by October 31.

Johnson's dramatic rise to Britain'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 lawmakers have vowed to thwart a no-deal Brexit.

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

"We have a momentous task ahead of us," Johnson, 55, told the first meeting of his cabinet at No. 10 Downing Street.

"At a pivotal moment in our country's history we are now committed, all of us, to leaving the European Union on October 31 or indeed earlier, no ifs, no buts," Johnson said at the opening of the meeting.

Johnson'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 the divorce deal that his predecessor Theresa May agreed last November but failed to get ratified.

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

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.

Johnson began his time in office by decisively sweeping away May's cabinet in one of the biggest culls of senior government jobs in recent British history.