Parents who lose a child will also be given rights to bereavement leave and people with fluctuating mental health conditions such as depression will be protected from discrimination under reforms to the Equalities Act, which now covers only constant conditions.
Issuing a series of challenges to the Tories over their plans for the economy, the party's economic spokesman Patrick O'Flynn said there was "still a stench in the political air" over favouring the rich and powerful despite Mrs May's recent bids to make gains among Labour and Ukip voters.
Prime Minister Theresa May promised to expand the rights of workers, boosting protection for those in the "gig economy".
The manifesto will include measures to protect workers' pensions in the wake of the BHS scandal and a guarantee European Union rights will be protected in the Brexit process. "We intend to meet that commitment by 2020 - and then continue with a national living wage linked to 60% of median earnings".
"The Conservatives will guarantee all rights that workers now enjoy as we leave the European Union", May will say Monday while visiting a training facility in the south of the United Kingdom, according to excerpts from her speech released early to the press.
The package of reforms includes changes to the Equalities Act to extend protections from discrimination to those suffering fluctuating or intermittent mental health conditions.
Labour campaign chief Andrew Gwynne said the Prime Minister was "taking working people for fools".
Vince Cable, the Liberal Democrat shadow chancellor, was equally scathing.
"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".
Admittedly, being the most worker-friendly Tory prime minister in history is a low bar to jump - May's claim that the Conservatives have always been the "true party" of workers is laughable.
Labour has committed to a so-called real living wage of £10 per hour by 2020.
But Mrs May's proposals were given a cautious welcome by the Trade Union Congress, which described the measures as a "promising set of commitments", but said workers needed to see the details before they cast their votes.
Edwin Morgan, deputy director of policy at the IoD, said the plan to give companies options on how to increase the voice of employees is a "very laudable aim", but the group added: "While it might be right for one business to have an elected worker representative, that might not work for every firm".
"The choice is clear: economic stability and a better deal for workers under my Conservative team, or chaos under Jeremy Corbyn".