The TUC, the umbrella group for trade unions in the UK, also urged the government to “stay on course” in delivering the bill.
Its general secretary Paul Novak told the BBC: “I am confident that the government will deliver on its commitments to enact that employment rights bill in full.”
Concern is coming from unions on the right of the trade union movement, not the usual suspects who might be expected to criticise the government.
Mr Novak said the bill “will give a massive boost to rights for millions of working people in this country, lots of whom are in insecure, low-paid employment”.
“My message to the government is, stay on course, deliver the employment rights bill, and deliver it in full,” he said.
He also pushed back on the idea the government could make concessions to business in the legislation.
“I think that’s the last thing government should countenance”, Mr Novak said.
“Having people in well-paid, secure employment is good for everybody. It’s good for workers, but it’s also good for the good employers who are being undercut by the cowboys, and it’s good for the UK economy.”
The USDAW union represents more than 300,000 workers, many of whom work in supermarkets, factories and warehouses.
The union is concerned that an amendment to the Employment Rights Bill, which was added by the House of Lords in July, could be adopted by the government.
The amendment seeks to change the bill from a requirement for an employer to offer guaranteed hours to a right to request guaranteed hours by an employee.
The union is concerned that this would be a weakening of their members’ rights because there would no longer be a right to a contract that reflects their normal hours of work.
The Federation of Small Businesses (FSB) has told the BBC that Angela Rayner’s resignation and the government reshuffle present an opportunity to “fix the issues” with the Employment Rights Bill.
“In some of our recent polling of members, 92% of employers said they were worried about this legislation. We have never got to that level in a poll,” said Craig Beaumont, director of policy at the FSB.
“They are overwhelmed by the changes that are coming. They don’t have HR teams. All these different measures scare them, and this is a chance to fix the issues.”
Mr Beaumont said that Rayner’s resignation and the sacking of employment minister Madders – two key government supporters of the bill – could allow for some compromise.