Political parties are readying themselves for a general election campaign after MPs voted for a 12 December poll.
The legislation approved by MPs on Tuesday will later begin its passage through the House of Lords, where it is not expected to be opposed.
Boris Johnson says he is ready to fight a "tough" general election.
Labour leader Jeremy Corbyn said the snap poll was a "once-in-a-generation" opportunity to transform the country.
Writing in the Daily Mirror, he said: "We're launching the most ambitious and radical campaign for real change that our country has ever seen."
But Mr Johnson hopes the vote will give him a fresh mandate for his deal to leave the EU and break the current deadlock in Parliament.
He told Conservative MPs it was time for the country to "come together to get Brexit done", adding: "It'll be a tough election and we are going to do the best we can."
The poll comes after the EU extended the UK's exit deadline to 31 January 2020 - although Brexit can happen earlier if a deal is agreed by MPs.
Mr Corbyn and Mr Johnson will face each other at Prime Minister's Questions at mid-day - likely to be final clash before Parliament is dissolved for the election.
What happens next?
The Early Parliamentary General Election Bill - which prompts the election - will be debated in the House of Lords on Wednesday
If peers make any amendments to the bill, it will head back to the Commons for MPs to approve or reject the changes
Once passed, the bill will receive Royal Assent - when the Queen formally agrees to the bill becoming law
On Monday 4 November, MPs are due to elect a new Speaker to replace John Bercow
Just after midnight on Wednesday 6 November, Parliament will be shut down or "dissolved" - meaning every seat in the House of Commons becomes vacant
Five weeks later, the country will go to the polls for the first December election since 1923
The legislation approved by MPs now must be rubberstamped by the House of Lords.
It would be pretty strange if unelected peers up the corridor from the green benches decided to say no or throw spanners in the works of a decision made by the House of Commons last night.
Unless something very strange happens, we are now on for an election.
Both sides are very, very nervous about what might unfold. And both sides are right to be nervous.
The two main party leaders, in a strange kind of mirror of each other, are happy campaigners, but divisive characters.
Both of them will try to set the agenda, but they can't know where this will all take us.
They can't know if it will be their issues they're able to talk about at length, but that's the glory of elections - it's up to voters to set the terms.
They decide the things they care about, they are interested in and they will put politicians on the spot about.
Labour's Shadow chancellor John McDonnell said the election wouldn't just be about Brexit - telling BBC Breakfast: "It will be about what has happened over the last nine years of austerity and our public services."
"We want to give people hope again about the future," he said.
Health Secretary Matt Hancock said he was "certainly not yearning for general election", but he believed it was "the only way to move the country forward".
He said the Conservatives would be offering an "centrist agenda" to voters - and his party would be running a more "optimistic" campaign than their widely-criticised 2017 effort, which led to Theresa May losing her majority.
Liberal Democrat leader Jo Swinson said the poll was "our best chance to elect a government to stop Brexit".
For the Scottish National Party, Scotland's First Minister Nicola Sturgeon said an election was an opportunity for the country to hold another independence referendum.
"A win for the SNP will be an unequivocal and irresistible demand for Scotland's right to choose our own future," she said.
But the Scottish Conservatives claimed voting for their party would keep Scotland in the UK.
Brexit Party leader Nigel Farage welcomed the election, tweeting the deadlock had been "broken" and "Brexit now has a chance to succeed".
Electoral pacts and coalitions
The smaller parties are already talking about striking deals with each other, which could see candidates standing aside in areas where their Remain-supporting rivals have a better chance of winning.
Deputy Lib Dem leader Ed Davey said the party was in discussions with Plaid Cyrmu and the Green Party "to see if we can work with them".
Green Party co-leader Jonathan Bartley told BBC Breakfast it was "no secret" that the Greens were "talking to the Lib Dems and Plaid" but "nothing has been finalised".
He said a "temporary electoral arrangement to stand aside for one another" would help elect "a big block of MPs that aren't from the two main parties".
"That idea of getting a big block of MPs in parliament who want a people's vote, want to remain in the EU because they think it's best for the UK to do that, has widespread appeal."
The party leaders are also facing questions about whether they would form a coalition, if the election resulted in another hung Parliament.
Asked if she would form a coalition government with Labour or the Conservatives, Jo Swinson said: "I can't be clearer - neither Boris Johnson nor Jeremy Corbyn is fit to be prime minister."
Labour's John McDonnell told BBC Breakfast: "There will be no deals, no coalition. We will go in as a majority government. If we don't win an overall majority, we will have a minority government."
How did we get here?
After months of deadlock over Brexit and three previous attempts, Boris Johnson finally managed to get MPs to back his call for a general election.
It came after the SNP and Lib Dems broke ranks with Labour and indicated they would be prepared to back a poll, having previously worked together to frustrate Mr Johnson's efforts.
Labour leader Jeremy Corbyn announced on Tuesday morning that he would back an election because of the EU's decision to grant a three-month Brexit extension to 31 January meant no-deal had been taken off the table.
The government then tabled a one-page bill proposing a 12 December election to the Commons which needed a majority of just one, unlike previous efforts which needed a majority of two-thirds of all MPs.
Labour's amendment to change the date of the proposed election from 12 to 9 December was rejected and MPs voted to back the government's original bill by 438 votes to 20.