The government has introduced what it claims to be the biggest package of workplace reforms for 20 years after concerns that ministers have failed to appeal to voters who are “just about managing”.

Legislation that comes into effect on Monday will increase fines for employers who have deliberately victimised their staff, and give workers details of their rights from the first day in their job.

The reforms, outlined by the business secretary Greg Clark, plan to stop the “unfair” use of pay-between-assignment contracts which allow businesses to opt out of equal pay arrangements for agency employees. Measures will also be taken to ensure that seasonal workers get the paid time off they are entitled to.

The reforms will be seen as a direct appeal to Labour voters and an attempt to get back to the values expressed by Theresa May in July 2016. In her first speech as prime minister, May said: “If you’re from an ordinary working class family, life is much harder than many people in Westminster realise. You have a job but you don’t always have job security.” She pledged to protect those who were “just about managing”.

Labour has questioned whether the plans will do anything for people on zero-hours contracts. Unions said the reforms were a missed opportunity to rebalance power in the gig economy.

The reforms, covered in a report called the Good Work Plan, are based on the recommendations made by former Tony Blair adviser Matthew Taylor in his review into employment, especially in the so-called gig economy. The government has pledged to implement 51 of Taylor’s suggested 53 reforms.

Legislation will be introduced to give workers details of their rights from the first day in a job, such as eligibility for sick leave, pay levels, maternity and paternity leave.

The maximum employment tribunal fines for employers demonstrated to have shown malice, spite or gross oversight will increase from £5,000 to £20,000.

Ministers are also committing to legislate to improve employment status tests to “reflect the reality of the modern working relationships”. Despite calls from unions, the government said the reforms would not ban zero-hours contracts, in keeping with Taylor’s recommendations, because that would “negatively impact” more people than it helped.

TUC general secretary Frances O’Grady welcomed some reforms such as the decision to scrap the so-called “agency worker loophole”.

However, Grady said the government had missed an opportunity to bolster the rights of zero-hours workers. “These reforms as a whole won’t shift the balance of power in the gig economy. 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.

“The right to request guaranteed working hours is no right at all. Zero-hours contract workers will have no more leverage than Oliver Twist,” she said.

Rebecca Long Bailey, shadow business secretary, said the reforms will do little to combat the government’s attacks on workers’ rights such as an increase in employment tribunal fees, the Trade Union Act and low wage growth.

“These proposals do nothing to tackle the growing number of people on precarious zero-hours contracts and with their botched Brexit deal threatening jobs and rights they’ll have to do a lot more than this to reassure workers,” she said.

The government claims the reforms will make the UK the first country in the world to address the opportunities and challenges of the gig economy and its impact on a modern economy.

They form a key part of the government’s modern industrial strategy, a long-term plan to help businesses create better, higher-paying jobs in every part of the UK.

Clark said: “The world of work is changing, bringing new opportunities for innovative businesses and new business models to flourish, creating jobs across the country and boosting our economy.

“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.”

Taylor told the Guardian: “I think this is a welcome and substantive response from the government and will make a real difference to the working lives of millions of workers, particularly those in the most vulnerable positions.”

One business representative broadly welcomed the suggested reforms. Matthew Fell, CBI chief UK policy director, said: “Businesses support a strong floor of workplace rights, and it’s right that these laws keep pace with changes in the economy and society.

“They 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.

“However, legislation to amend employment status rules risks making the law less able to adapt to new forms of work in the future,” he said.