Unions have accused ministers of giving an "open invitation" to bad bosses to exploit workers with their plans to tear up EU protections.
The Government said it would consult on slashing "unnecessary red tape on recording working hours", stoking fears workers could be forced to do longer shifts.
Union leaders said there was a danger people will be forced into excessive hours and said it was a "recipe for low-paid, burnt-out Britain".
The move comes despite Boris Johnson repeatedly vowing that Brexit would not undermine protections for employees.
Rishi Sunak also sparked Tory fury by ditching his end-of-year deadline for a "bonfire" of EU laws in a move branded a "humiliating U-turn" by Labour.
Ministers will now revoke only 600 laws under the Retained EU Law Bill - rather than the 4,000 bits of legislation originally promised.
The Government has been looking at overhauling EU rules protecting workers to save businesses cash.
Business Secretary Kemi Badenoch said in a written statement: "As part of this drive for deregulation, today I can announce that we will make improvements to employment law which could help save businesses around £1 billion a year, while safeguarding the rights of workers.
"We will consult on cutting unnecessary red tape on recording working hours, streamline engagement with workers when a business transfers to new owners, and provide up to 5 million UK workers greater freedom to switch jobs by limiting non-compete clauses."
The move sparked fury from union leaders, who said workers were being betrayed the plans.
TUC General Secretary Paul Nowak said: "People are already working all hours to make ends meet.
"Paid holiday and safety measures like rest breaks and limits on excessive hours are all fundamental protections - not a nice-to-have.
"This is a recipe for low-paid, burnt-out Britain. Yet this Conservative Government was elected on a promise to make this country the best place in the world to work."
Unite General Secretary Sharon Graham said: “The Government's so-called attack on red tape is nothing more than a boost for bad bosses and will pile more misery on workers.
“Removing the requirement for employers to record workers’ hours, means that the regulations effectively become unenforceable.
“Unscrupulous employers will use these emasculated regulations to exploit young or unorganised workers. Cutting red tape? More like a playbook for profiteering.”
Unison General Secretary Christina McAnea said: "Ditching measures that keep workplaces safe and ensure staff are treated fairly is not a recipe to grow the economy, nor level up disadvantaged communities.
"Workers were promised a better deal following Brexit. This is anything but. It's nothing less than an open invitation to underhand employers to exploit and mistreat as they see fit."
Ross Holden, GMB Research and Policy Officer, said: “This is the latest attack on working people by the Tories who have long wanted to turn the clock back on our hard-fought rights.
“The dangers of working excessive hours to health and safety is well known.
“This isn’t about removing ‘red-tape’ – it’s about removing our rights to come home from work safe, to go to work rested and to have proper time off to spend with our families.
“Workers are already paying the price for the Conservatives crashing the economy. Now they could be forced to work longer when its unsafe and unhealthy to do so.”
Labour hit out at the Government's decision to water down its long-touted post-Brexit bonfire.
Shadow Cabinet Office Minister Jenny Chapman said: “This is a humiliating U-turn from a weak and divided government with no clue how to grow our economy, protect workers, support business or build a better Britain outside the EU.
“After wasting months of parliamentary time, the Tories have conceded that this universally unpopular bill will damage the economy, at a time when businesses and families are already struggling with the Tory cost of living crisis.
"They are now trying to adopt some of Labour's amendments to try and rescue this sinking ship of a bill.”
Former Cabinet Minister Jacob Rees-Mogg tweeted: "Regrettably the Prime Minister has shredded his own promise rather than EU laws."
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