Hundreds of thousands of workers face having their pay and conditions hit under changes to employment law covering staff who transfer to different employers, it has been warned.
The TUC said protections to pay and conditions will be "substantially weakened", and job security hit under moves coming into force today.
Low paid workers in contracted-out services such as cleaning, catering and social care will be particularly hard hit by changes to the Transfer of Undertakings and Protection of Employment (Tupe), said the union organisation.
Employers could be encouraged to cut pay and conditions and sack staff after a transfer takes place under changes to collective agreements, said the TUC.
General secretary Frances O'Grady said: "Watering down Tupe law means hundreds of thousands of vulnerable workers around the UK will lose out on vital protections at work. This is a deliberate bid by government to make privatisation cheaper and quicker.
"Weakening guarantees on pay and conditions will encourage unscrupulous companies to compete for contracts based solely on low costs and not on the quality of service.
"The government is sending a clear signal to employers that the rights and pay of low-paid workers are not important at a time when living standards are being hugely squeezed."