Jeremy Corbyn is expected to accuse "unscrupulous bosses" of driving down wages and undermining workers' rights.
At a speech to the Trades Union Congress (TUC) on Tuesday (12 September), the Labour Party leader will set out Labour's "job first" plan for Brexit, a day after his party was criticised for failing to present a united front over the EU withdrawal bill, which was passed with a majority of 36.
"We must never let ourselves be duped and divided," Corbyn will say.
"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."
Corbyn is also expected to draw attention to the risks of the so-called "gig economy", arguing it has had a deteriorating effect on the mental health of a lot workers.
While "gig economy" companies present their industries as "modern and dynamic", they deny "both employees and customers basic protections", he is due to say. Businesses relying on short-term and temporary contracts are also expected to be targeted by the opposition leader, who will describe them as a "source of continuous worry and insecurity for millions of people".
Corbyn is also expected to urge young people attending the traditional TUC conference in Brighton to ignore the way trade unions are "demonised" and to ensure their rights are protected by joining one.
The Labour leader will add that winning the General Election alone would not be enough unless it was supported by higher participation in trade union bodies.
"Politics is about power and democratic politics is about putting power in the hands of the many not just the few," he is expected to say. "That principle applies in the workplace too."
Corbyn's speech comes a day after Unite general secretary Len McCluskey warned there was a real chance co-ordinated strikes on public sector pay would go ahead. Since March, strike ballots require a minimum 50% turnout to go ahead but McCluskey said he was prepared to carry out industrial action if members of the union backed a strike over pay, even without reaching the legal threshold.
The TUC conference has been focussed largely on the issue of public sector pay. On Monday (11 September), the body General Secretary, Frances O'Grady, said public workers had "had enough" after seven years of pay freezes.
Public sector pay was frozen for two years in 2010, except for workers earning under £21,000 per year, and the increase has been capped at 1% since 2013, below the rate of inflation. It is understood the government will lift the cap for police forces and prison officers, but O'Grady warned against "cherry-picking" particular categories, insisting the whole sector deserved a pay rise instead.
"Five million hardworking public servants need it," she said. "The public backs it. Now just tell the Treasury to get on with it and give public sector workers a pay rise - no cherry-picking. All public services deserve a pay rise, and they deserve it now."