The electorate, also deeply divided over Brexit, will now have its say in a poll dubbed the “most important” for a generation.
Here is what you need to know.
Why is an election being held?
Months of fractious political disagreement over Brexit saw MPs agree in October to hold an early general election.
Before that, they had repeatedly rejected the divorce deal brokered by Johnson‘s predecessor, Theresa May, forcing her resignation. They then moved to block Johnson from pushing his revised Withdrawal Agreement through Parliament at breakneck speed.
With no apparent majority among MPs for any course of action over Brexit – resulting in three requests so far to delay the UK’s departure – an election appeared to be the natural end-game as the year progressed.
“What has defined this period is an inability to make decisions and carry out decisions with enough support,” Alex Stojanovic, a Brexit researcher at the UK’s Institute for Government, told Al Jazeera.
“That’s why there has been chaos – because there are fundamental splits about how we should proceed and no-one has been able to resolve those splits.
“We needed an electoral event to see if it can break the deadlock by changing the [parliamentary] calculus.”
How does the vote work?
Voters in 650 constituencies across the UK will elect an MP to the lower chamber House of Commons via the first past the post system. To win, candidates need to get more votes than any of their competitors.
A party needs to win 326 seats to secure a majority in the Commons and be asked to form a government by the monarch, Queen Elizabeth II. Parliament’s upper chamber, the House of Lords, is unelected.
If no party achieves a majority, there is a hung parliament. In this scenario, the party with the largest vote share may form a minority government, seek out the support of smaller parties for a “confidence and supply” arrangement, or try to build a formal coalition.
On polling day, voting centres will be open from 7:00 – 22:00 GMT. Final results from the poll are expected to be declared by the early hours of December 13.
What is expected?
The Conservative Party appears set to win the largest vote share of the vote and possibly a majority of seats.
The party is nine percentage points clear of its nearest rivals, Labour, with 43 percent of voters saying they would vote Conservative if an election were held tomorrow, according to the latest data compiled by market research and data analytics firm YouGov.
That could translate into the Conservatives winning 359 seats in the Commons, according to YouGov polling, compared to 211 for Labour.
Among the smaller parties, the pro-European Liberal Democrats are predicted to win 14 percent of the vote share and 13 seats in Parliament. The Scottish National Party (SNP), which only fields candidates for the 59 constituencies in Scotland, is projected to win 43 seats, but only three percent of all votes cast.
John Curtice, professor of politics at Strathclyde University and senior research fellow at NatCen Social Research, said recent polls reflected the “primacy” of Brexit.
“Much of the movement in the campaign has been within the two Brexit camps [leave and remain] rather than between them and there is certainly little sign of the ability of parties to be able to gain votes across the Brexit divide,” Curtice told Al Jazeera.
“The increase in Conservative support during the election campaign has occurred among leave voters, it has not increased its support among remain voters at all, and Labour has barely done any better amongst leave voters,” he added.
How will the vote shape Brexit?
Johnson has promised to “get Brexit done” by the end of January if his Conservative Party scoops a majority.
However, even if Parliament passes the existing withdrawal agreement by that deadline, a potentially gruelling negotiation over the UK and EU’s future relationship will begin as the transition period comes into effect.
Johnson says this period will not be extended past the end of 2020, when it is currently scheduled to end, but there is widespread doubt over whether a trade deal can be concluded before then.
The transition period can be extended by up to two years if the UK and EU both agree.
If Labour wins, it will ask the EU for another Brexit extension to allow time to renegotiate the current withdrawal agreement.
Corbyn wants a softer divorce deal based on a new UK-EU customs union and close EU single market alignment.
Labour’s reworked agreement would be put to a legally-binding second referendum within six months, alongside an option to remain in the EU.
The SNP is aligned with Labour in supporting a second referendum, so long as remaining in the EU is an option on the ballot paper, while the Liberal Democrats have pledged to revoke Article 50 and cancel Brexit altogether if they win.
Regardless of the outcome, Stojanovic said, there is likely to be “more uncertainty and more difficult, fractious negotiations ahead”.
“The Brexit question, no matter what happens, will probably be unresolved … for a long time to come in British politics,” he said. “It has opened up fundamental questions about the direction of the country and where it wants to go.”
What else are Britons concerned about?
Uncertainty over the UK’s future relationship with the EU has seen the economy register its slowest annual rate of growth in almost a decade, with year-on-year growth in the last quarter falling to an anaemic one percent.
Meanwhile, the NHS continues to struggle under severe financial pressure, with hospital waiting times now at their worst-ever level on record.
A decade of austerity imposed by successive Conservative-led governments has also seen budgets for policing, housing and welfare all dramatically cut.
Johnson and Corbyn have offered vastly different visions for how to reshape the UK.
Johnson has pledged to borrow to spend on a major infrastructure investment programme, increase NHS funding and put 20,000 more police officers on the streets.
“The Conservatives’ manifesto is very modest,” Stojanovic said. “There aren’t that many big pledges, partly because Brexit is the big pledge – that is the radical thing they are doing.”
Corbyn, by comparison, has promised a fundamental shake-up of the UK economy, including increasing tax and spending, and a nationalisation programme.
Labour’s economic proposals are largely popular among the electorate, YouGov polling indicates, but many are cool on Corbyn’s leadership, Curtis said.
“Labour’s problem is not that it has got to sell its ideas, it is that it has got to sell its ability to deliver,” he added. “That is something that is quite difficult to change in a matter of weeks.”