Workers on zero-hour contracts, agency employees or "gig economy" workers are to be better protected by a package of workplace reforms, the government says.
Under new legislation to be introduced on Monday, staff would have to be told details of their rights from their first day in a job, including eligibility for paid and sick leave.
Workers would also be given the right to request more predictable hours.
But Labour and the unions say the planned reforms do not go far enough.
The reforms are based on the findings of a review into modern working practices led by Matthew Taylor, a former aide to ex-PM Tony Blair and chief executive of the Royal Society of Arts.
Ministers say the new legislation will:
- close a loophole that had allowed agency staff to be paid less than permanent employees
- ensure firms will have to provide a "statement of rights" on the first day of a person's employment, setting out what paid leave they are entitled to, including for illness, maternity and paternity leave
- increase the maximum fine employers face at a tribunal from £5,000 to £20,000, if they are found to have demonstrated "malice, spite or gross oversight"
- ensure that companies will have to calculate holiday pay based on 52 weeks, as opposed to 12 weeks, so people in "seasonal or atypical roles get the paid time off they are entitled to"
Business Secretary Greg Clark said: "Today's largest upgrade in workers' rights in over a generation is a key part of building a labour market that continues to reward people for hard work, that celebrates good employers and is boosting productivity and earning potential across the UK."
However, Frances O'Grady, general secretary of the Trades Union Congress, said: "The right to request guaranteed working hours is no right all.
"Zero-hours contract workers will have no more leverage than Oliver Twist. "
"Unless unions get the right to organise and bargain for workers in places like Uber and Amazon, too many working people will continue to be treated like disposable labour," she added.
Matthew Fell, chief UK policy director at the CBI, said: "[Businesses] welcome a new law giving all workers the right to request more predictable working hours, which will help to facilitate the conversations that are essential to ensuring flexibility benefits both parties.
But he said: "Legislation to amend employment status rules risks making the law less able to adapt to new forms of work in the future."
The government agrees with the Taylor review that completely banning zero-hour contracts "would negatively impact more people than it helped".
It also said that platform-based working - including the likes of ride-sharing app Uber or food delivery business Deliveroo - offers "genuine two-way flexibility" for firms and workers.
Zero-hour contracts, where an employer does not have to offer work and an employee has no obligation to work when asked, are highly controversial.
The Archbishop of Canterbury called the contracts "the reincarnation of an ancient evil".