The former London mayor easily beat his rival, Foreign Secretary Jeremy Hunt, in a poll conducted among members of the governing Conservative Party.
He is expected to be confirmed as prime minister on Wednesday, when his predecessor Theresa May formally tenders her resignation to Queen Elizabeth II.
It is a triumph for a man who has always wanted the top job, but Johnson, known for his jokes and bluster, is taking over at a time of immense political upheaval.
Three years after the referendum vote to leave the European Union, Britain remains a member, after twice delaying its exit amid continued wrangling in a divided parliament -- and the country -- on how to proceed.
Johnson led the 2016 Brexit campaign and has said the latest deadline of October 31 must be kept, with or without a divorce agreement with the EU.
But Brussels says it will not renegotiate the deal it struck with May, which was designed to ease the end of a 46-year partnership but which MPs have rejected.
A majority of lawmakers are against a "no deal" Brexit, including many of Johnson's colleagues.
Three cabinet ministers have already said they will not serve under Johnson, saying that severing ties with Britain's closest trading partner with no new arrangements is deeply irresponsible.
The government's official forecaster said last week that Britain will slide into a year-long recession should it leave the EU without a deal.
Boris Johnson is expected to be elected leader of Britain’s governing Conservative Party and the next prime minister on Tuesday, tasked with following through on his “do or die” pledge to deliver Brexit in just over three months time, reports AzVision.az citing to Reuters.
Johnson and his rival, Foreign Secretary Jeremy Hunt, have spent the last month crossing the country seeking to win over the fewer than 200,000 Conservative Party members who will choose Britain’s new leader.
The winner will be announced at about 1045 GMT at an event near parliament and will formally take over as prime minister on Wednesday afternoon, succeeding Theresa May, who stepped down over her failure to get parliament to ratify her Brexit deal.
Johnson, a former London mayor who resigned as foreign minister a year ago over May’s Brexit plans, is the clear favorite to replace her, with several polls putting him on around 70 percent.
“It’s all to play for,” Johnson told reporters as he arrived at his office in Westminster.
He will inherit a political crisis over Britain’s exit from the European Union, due to take place on Oct. 31. Johnson must persuade the EU to revive talks on a withdrawal deal that it has been adamant cannot be reopened, or else lead Britain into the economic uncertainty of an unmanaged departure.
The only divorce deal on the table has been rejected three times by parliament and many lawmakers - including pro-EU rebels in the Conservative Party - are vowing to block Johnson from trying to take Britain out of the EU without a deal.
He has said he would ramp up preparations for a no-deal to try to force the EU’s negotiators to make changes to the accord.
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