Jeremy Corbyn is to hit out at "unscrupulous" employers who use new technology to deny their staff basic protections in the workplace.
The Labour leader will use his address to the annual TUC Congress in Brighton to link the rise of the "gig" economy to the country's worsening mental health.
He will point to research by Manchester University which found that poor quality jobs were actually worse for mental health than being unemployed.
He will urge young workers to ignore the way unions are "demonised" in the press and to safeguard their rights by joining one.
"If you want a job that pays a decent wage, gives you the chance to get on in life, live independently and enjoy your work, then join a trade union. Do it today," he will say.
"I know it's a shock that billionaire tax-dodging press barons don't like trade unions. They don't like us because our movement challenges unaccountable power of both government and bosses."
Mr Corbyn will say that while the spread of "flexible" employment through the use of new technology platforms was supposed to benefit workers as well as employers, for many it was a source of perpetual anxiety.
"Increasingly flexible employment is sold to us as a benefit, they call it the gig economy, and who doesn't like going to a gig?" he will say.
"It is a benefit to unscrupulous employers but it is the source of continuous worry and insecurity for millions of people and is in part responsible for the worsening mental health of a country that has lost over 6,000 mental health nurses in recent years.
"The 'gig economy' presents itself as modern and dynamic, but all too often uses technology as a cover to deny both employees and customers basic protections."
Mr Corbyn will reiterate Labour's commitment to a "jobs-first Brexit" that maintains and develops workers' rights and consumer and environmental protections after Britain leaves the EU.
He will say that once EU free movement rules end, a Labour government would bring in a system of "reasonable" management of migration based on the needs of the economy.
"As we pledged at the election, we want to see them replaced by fair rules and reasonable management of migration that put jobs, living standards and the economy first, not fake immigration targets, as the Tories do, that will never be met," he will say.
"We must never let ourselves be duped and divided. It isn't migrants who drive down wages and conditions but unscrupulous employers, supported by a government that slashes rights and protections at work whenever it gets the chance."