Ed Miliband and senior Labour figures have demanded employment regulation reforms that are feared to hit low-paid workers undergo further examination.
The Labour leader has tabled a motion in the House of Commons asking for amendments linked to employment law covering staff who transfer to different employers to be annulled, which is a procedural mechanism that can trigger extra scrutiny or a debate.
In January, changes were introduced to the transfer of undertakings and protection of employment (Tupe) regulations without the need for a debate or a vote.
Shadow chancellor Ed Balls, shadow work and pensions secretary Rachel Reeves, shadow business secretary Chuka Umunna and shadow justice secretary Sadiq Khan have backed Mr Miliband's proposal, which seeks greater scrutiny from MPs over the measures.
Employment relations minister Jenny Willott has previously insisted the changes to Tupe will give businesses greater clarity about conducting transfers, while also "protecting fairness for all" and ensuring "appropriate levels" of protection for employees.
But the Trades Union Congress (TUC) believes the alterations will affect the pay and conditions of hundreds of thousands of workers, claiming it will also hit job security and low-paid workers hard.
A Labour source said: "We are praying against the changes as there has not been significant parliamentary scrutiny. We hope this will enable more scrutiny in detail."
Frances O'Grady, TUC general secretary, said: "Watering down Tupe law will make it easier for private companies to compete to take over our public services by lowering staff costs and boosting their profits, which will reduce the quality of services that taxpayers use and pay for.
"These new regulations will increase poverty pay - particularly for staff in catering, cleaning and social care - at a time when living standards are already being squeezed.
"They should be scrapped immediately."
The Labour MPs who signed the early day motion, in alphabetical order, are Diane Abbott, Ed Balls, Roberta Blackman-Woods, Martin Caton, Jeremy Corbyn, Jim Dobbin, Paul Flynn, Huw Irranca-Davies, Sadiq Khan, Sir Alan Meale, Ed Miliband, Ian Murray, Steve Reed, Rachel Reeves, Steve Rotheram, Graham Stringer, Chuka Umunna and Rosie Winterton.
Green Party MP Caroline Lucas and Mark Durkan, of the Social Democratic and Labour Party, have also given their support.