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Its manifesto will promise to keep all workers' rights currently guaranteed by EU law, put worker representation on company boards and protect pensions from "irresponsible" bosses.
Economic growth would be "spread to everyone in our country", Mrs May said.
But Labour said she was "taking working people for fools".
And the GMB union said its members would "believe it when they see it".
In what BBC political correspondent Ben Wright says is an "unabashed pitch for Labour voters", Mrs May will promise a "new deal for workers", with a string of proposals.
These include a statutory right to leave for those who need to care for a family member, and for parents who have lost a child.
There would also be new protections for people in the "gig economy", a statutory right to training, measures to protect workers' pensions in the wake of the BHS scandal, and a guarantee that European Union rights will be protected in the Brexit process.
The national living wage for workers aged 25 and over, currently £7.50 an hour, would rise in line with average earnings until 2022.
The Tories would also insist listed companies had representation for workers on their boards - whether on advisory panels, as a non-executive director or through a directly appointed worker representative - although firms would not be forced to have actual employees in the boardroom.
Mrs May will say: "By working with business, reducing taxes and dealing with the deficit, we have delivered steady improvements to the economic prospects of working people.
"Now is the time to lock in that economic growth and ensure the proceeds are spread to everyone in our country."
Labour, which has repeatedly warned of the threat to workers' rights posed by Brexit, dismissed the announcement.
It argued the Tories had overseen an "era of non-compliance of employment law", an "explosion in low pay and stagnating wages" and a "massive expansion in bogus self-employment".
It also said a pledge from Mrs May to put workers on company boards had been "watered down".
"Theresa May and her Tory government have failed to stand up for workers, with hundreds of thousands not being paid the money owed to them, thousands unable to get their case against their employer heard, and hundreds of complaints of employment agency malpractice going un-investigated," said campaign chairman Andrew Gwynne.
Vince Cable, the Liberal Democrats' former business secretary, said: "The Conservatives tried to ban workers from striking and were blocked by the Liberal Democrats in government.
"It's clear they aren't the party of workers' rights and that you can't trust them to care about you and your family."
GMB general secretary Tim Roache said achieving the greatest expansion of workers' rights by a Conservative government "wouldn't be that hard to achieve given recent history".
He called for an end to the squeeze on public sector pay and to the "wide scale abuse" of agency contracts.