This week’s European Union withdrawal bill will be a key moment for everyone concerned about fair treatment in the workplace. It’s the government’s vehicle to write EU law into UK law – but if that sounds routine, it’s anything but. Among the laws being transferred are those workers’ rights which trade unions have won from our membership of the EU: the right to paid holidays, health and safety protections, unpaid parental leave, time off for family emergencies, equal pay for women, and equal treatment rights for part-time, fixed-term and agency workers.

It is true that the government has promised to protect all the workers’ rights that come from the EU. But we are very concerned that the bill won’t do enough to protect those rights effectively. Hidden in the bill are mechanisms that allow future governments to gut some of those hard-won rights. It would give imaginative lawyers acting for unscrupulous employers the opportunity to whittle down our rights through endless legal challenges. Most at risk are the rules that protect small groups of vulnerable workers – which are far easier to pick off.

The gig economy and zero-hours contracts have shown that laws haven’t kept pace with the ways bosses can exploit workers

Chief among these is holiday pay. Before these rules came in, in 1998, more than a third of part-time workers had no right to paid holidays. Any undermining now of paid holiday for part-timers would hit more than 2 million workers. We are particularly worried about the rules ensuring that people who aren’t in full-time, permanent jobs get properly paid holidays. Employers often seek to undermine these rights – and unions have frequently had to go to court to make sure workers get their holiday pay in full.

Next on the hit list could be agency workers. Bosses lobbied hard to water down their protections, and still too few of them get the same pay as the regular staff they work alongside. EU protections have at least granted agency workers equal access to facilities like staff canteens, and some have received pay rises and more paid holidays.

Even Matthew Taylor’s lacklustre recent review on modern working practices advised the government to get rid of the loophole – known as the Swedish derogation – that causes most of the problems for agency workers. Theresa May still hasn’t acted on his recommendation. But if she ever does, some employers would want to recreate the loophole as soon as possible, and the bill could pave the way for that.

Bad bosses will also be keen to dilute the safeguards for workers when companies are restructuring, being taken over or making redundancies. EU rules ensure that management has to consult their workforces when big changes are being planned, and these TUPE rules protect workers from losing their jobs overnight when part of a company is sold off or a contract is transferred.

Under the EU withdrawal bill, changes to employment and equality law could be made through regulations, meaning MPs won’t get the chance to debate and discuss any potential change – so a future government could undermine rights without scrutiny. And while the rights we enjoy at work may now be transferred into British law at the point we leave the EU, the legislation doesn’t protect those rights beyond that point – meaning new laws or court judgments could put them at risk. With a government that’s shown itself unwilling to stand up to the employer lobby on top pay or workers on boards, we need stronger safeguards. In future, ministers should only be able to change the workers’ rights that came from the EU through primary legislation with full debate in parliament. And, to prevent British workers being treated worse than our continental neighbours, any eventual deal with the EU must include a commitment to a level playing field of workers’ rights.

No one could argue that the rights at work derived from the EU are good enough on their own. The advent of the gig economy and zero-hours contracts has shown that laws haven’t kept pace with the changing ways in which bosses can exploit their workers. Too few working people have a say over their wages and conditions at work through collective bargaining.

That’s why the TUC is calling for better terms and conditions for all workers in Britain. Brexit has to mean a new deal to strengthen union and workers’ rights. It is the best way to guarantee that workers don’t pay the price of Brexit.

• Frances O’Grady is TUC general secretary