Business groups and unions are urging the government not to go ahead with plans to ditch a wide range of EU laws, warning the move could cause "confusion and disruption" in the UK.
In a joint letter, groups including the Institute of Directors and the Trades Union Congress called on ministers to withdraw its Retained EU Law bill.
They warned the bill would put vital protections at risk.
Downing Street said it wanted to take "advantage of the benefits of Brexit".
The UK copied over the laws to smooth its exit from the EU on 31 January 2020, and kept them during a transition period that ended in January 2021.
Since then, the country has moved away from EU laws in certain areas, including on immigration, payments to farmers, and gene-editing rules for crops.
But thousands of regulations - known as "retained EU law" - remain in force.
Leading Eurosceptics, such as former business secretary Jacob Rees-Mogg, have given their support to the Retained EU law bill, which passed its second reading last month.
Crucially, the bill contains a "sunset clause" that means that, by the end of next year, some laws could simply expire automatically.
The letter, which was addressed to the new Business Secretary, Grant Shapps, said: "We are concerned that if passed into law, [the bill] could cause significant confusion and disruption for businesses, working people and those seeking to protect the environment.
"The bill would automatically sweep away thousands of pieces of legislation and established legal principles."
The groups warn that the bill could endanger important worker, consumer and environmental rights derived from EU law, including holiday pay, safe working hours and protection from discrimination.
Laws on the labelling of meat and eggs and the ban on the slaughter of seals are also at risk, according to the groups.
The letter also warns that scrapping the laws could put the UK "in breach" of the trade deal with the EU, which could in turn lead to additional tariffs that will negatively impact UK exporters.
"Making these changes will prove costly and bureaucratic and would undermine the certainty and stability workers and businesses need if the economy is to prosper," it adds.
Other signatories include human resources body the Chartered Institute of Personnel and Development (CIPD), the Employment Lawyers Association, Greener UK, Wildlife and Countryside Link and the Civil Society Alliance.
Ben Willmott, head of policy at the CIPD, told the BBC the bill risked creating "a huge amount of additional and unnecessary work" at a time when the focus should be on getting the UK economy growing.
A government spokesperson told the BBC: "The government is committed to taking full advantage of the benefits of Brexit, which is why we are pushing ahead with our Retained EU Law Bill.
"This will allow us to ensure our laws and regulations best fit the needs of the country, including making sure we continue to protect and enhance workers' rights and support jobs."