Labour leader Jeremy Corbyn will say a Conservative victory on 8 June is not a “foregone conclusion”, in his first big general election campaign speech.
Mr Corbyn will vow to change the direction of the election by “putting the interests of the majority first”.
He will attack wealthy tax avoiders and insist a Labour government will not “play by their rules”.
Theresa May is hoping to convert the Tories’ double digit poll lead into a bigger Commons majority.
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The prime minister says this will strengthen her hand in Brexit negotiations and provide the “strong and stable leadership” the country needs.
Her decision to hold a general election – after previously insisting she would wait until 2020 – took her rivals and many in her own party by surprise.
Mr Corbyn could have blocked it in Parliament but instead ordered his MPs to back the snap poll in a Commons vote on Wednesday.
The Labour leader looks set to run an anti-establishment campaign, presenting himself as a champion of the powerless against political and business elites.
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In a speech in London, Mr Corbyn will say: “Much of the media and Establishment are saying this election is a foregone conclusion.
“They think there are rules in politics, which if you don’t follow by doffing your cap to powerful people, accepting that things can’t really change, then you can’t win.
“But of course those people don’t want us to win. Because when we win, it’s the people, not the powerful, who win.”
He will add: “They say I don’t play by the rules – their rules. We can’t win, they say, because we don’t play their game.
“They’re quite right I don’t. And a Labour government elected on 8 June won’t play by their rules.”
He will add that those rules “have created a cosy cartel which rigs the system in favour of a few powerful and wealthy individuals and corporations”.
Mr Corbyn will say: “It’s a rigged system set up by the wealth extractors for the wealth extractors.”
The Labour leader will single out tycoon Sir Philip Green, who faced heavy criticism over the BHS pensions saga, and Sports Direct boss Mike Ashley in his speech, saying they should be “worried about a Labour government”.
Meanwhile, Mrs May sought to put “strong and stable” leadership at the heart of her pitch to voters as she addressed supporters in Bolton on Wednesday.
She said the country now has a “unity of purpose” and a desire for the government to “get on” with implementing Brexit and “making a success of it”.
The PM warned of a potential “coalition of chaos” led by Mr Corbyn, although the Labour leader ruled out forming a post-election alliance with the SNP.
Mrs May has faced criticism from rival parties for refusing to take part in head-to-head TV debates against other leaders during the campaign.
She said she preferred “to get out and about and meet voters”.
A Downing Street source said the PM would not appear on the same stage as another leader on television, but did not rule out an individual event with a studio audience.