Campaigning in the general election will officially begin later, after Parliament was dissolved in the early hours of Wednesday morning.
Boris Johnson will see the Queen at Buckingham Palace later to mark the start of the election period in the run-up to the 12 December poll.
The Conservatives will also launch their campaign, with Mr Johnson promising he can “get Brexit done”.
Meanwhile, in a speech Jeremy Corbyn will pledge “real change” under Labour.
Elsewhere, as the starting pistol is fired on five weeks of official campaigning:
- The Green Party will launch its campaign with a promise to invest £100bn a year on climate action
- The Liberal Democrats have pledged to spend £2.2bn a year on mental health services, funded by a 1% rise to income tax
- SNP leader Nicola Sturgeon will urge Remain voters in Scotland to back her party in a bid to stop Brexit.
Parliament was dissolved – or formally shut down – at just after midnight, meaning all MPs revert to being members of the public. Government ministers keep their posts.
The dissolution ended the shortest parliamentary session in just over 70 years, with the Commons having met for only 19 days since the state opening on 14 October.
Writing in the Daily Telegraph to launch the Conservative Party’s campaign, Mr Johnson compared his opponent Mr Corbyn to Soviet dictator Joseph Stalin
He said those in the party “point their fingers” at the rich “with a relish and a vindictiveness not seen since Stalin persecuted the kulaks” – wealthier peasants during the Russian Revolution, many of whom were murdered or starved to death.
Mr Johnson also likened the UK to a “supercar blocked in the traffic” by Brexit, adding: “If we can get Brexit done, there are hundreds of billions of pounds of investment that are just waiting to flood into this country”.
Mr Johnson – who will speak in the West Midlands later – said he did not want the election, but “we simply have no choice”.
“There is only one way to get Brexit done, and I am afraid the answer is to ask the people to change this blockading Parliament.”
Mr Johnson added: “It’s time to change the dismal pattern of the last three years and to get out of our rut.”
“The choice is clear. We can either go with Corbyn and his two favourite advisers, Dither and Delay… Or else we can vote for a sensible and moderate One Nation Conservative government”.
He repeated his claim that as well as another referendum on Brexit, a Labour government would also lead to a second vote on Scottish independence.
Mr Corbyn has previously said a new Scottish independence referendum was not “desirable or necessary” – but the SNP’s Nicola Sturgeon said she believed Labour would give the go-ahead for one if in government.
At his own campaign event in Telford later, Mr Corbyn is expected to say he would be a “very different kind of prime minister” who “only seeks power in order to share power”.
By BBC News political correspondent Jessica Parker
Parliament’s been dissolved and the official campaign period is now under way.
But Boris Johnson’s message, on the day of the Conservative Party launch, may already sound rather familiar.
He’s pledging to get Brexit done and deliver on a raft of spending priorities in, for example, education and the NHS.
And never shy of making waves the prime minister has, in the Daily Telegraph, accused the Labour Party of pointing their fingers at rich individuals with a vindictiveness akin to Stalin’s persecution of the kulaks; a class of affluent Russian peasants.
Jeremy Corbyn dismissed the comments last night as the kind of “nonsense” the super-rich come out with to avoid paying a bit more tax.
So, not exactly gentle opening exchanges as the campaign formally begins.
These two leaders do have something in common – they’re eager to be seen as itching to invest in public services.
But they do ultimately cut very different political figures and both appear ready, where possible, to try to point that out.
He will pledge to end in-work poverty and food bank use within five years – and said a future Labour government should be judged on whether it meets its promises.
“The politics I stand for is about sharing power and wealth with people who don’t have a lot of money and don’t have friends in high places,” he will say.
“My job as leader, and my party’s job, is to champion those people, and bring about real change.”
Kicking off Labour’s election campaign last week, Mr Corbyn said Labour “will put wealth and power in the hands of the many”.
Short session of Parliament: How does it compare?
- The 19-day session of Parliament this year is the shortest since October 1948, when MPs sat for just 10 days.
- That session was called purely as part of efforts to amend the Parliament Act to reduce the powers of the House of Lords.
- In February 1974, MPs sat for 60 days before Edward Heath’s Conservative government called a snap general election.
- A 1922 session of Parliament lasted only 17 days, as MPs met to pass legislation to approve the Irish Free State.
- A year earlier, there was a four-day session purely to approve the Anglo-Irish Treaty.
On Tuesday, the Liberal Democrats launched their campaign, with leader Jo Swinson saying the election could be a “a moment for seismic change” when “a new and different politics” emerges.
Ms Swinson said she could do “a better job” than either Boris Johnson or Jeremy Corbyn as prime minister.