Demonstrations will be held across the country for higher pay for young workers amid evidence that firms are cutting staff perks to pay for the National Living Wage.
The GMB union will stage protests in cities including Manchester, Birmingham and Cardiff, calling for an end to the "outdated" practice of younger employees being paid less than older workers.
The Living Wage of £7.20 an hour only applies to over 25-year-olds.
Food chain Eat has stopped paying staff for their lunch breaks, while employees at Caffe Nero are losing their right to a free lunch.
TUC general secretary Frances O'Grady said: "It's great that the vast majority of employers are managing to meet the higher minimum wage without cutting pay and conditions.
"If they can do it, then the minority of companies doing things like cutting paid breaks should think again. Employers using higher pay as an excuse to cut wider terms and conditions will be called out."
Ross Holden, of the GMB Young Members Network, said: "In the UK where age discrimination is ostensibly illegal, it is worrying that those aged under 25 can be paid up to 40% less than their over 25 colleagues for the same work, as a result of the Government's own living wage policy.
"Even the over 25s rate of £7.20 per hour falls well short of the £10 per hour rate that many need to make ends meet."
GMB national officer Kamaljeet Jandu said: "We are calling for companies to pay workers £10 an hour and stop this outdated practice of underpaying young workers."