More than 500,000 young people suffer wage discrimination because they are paid less than the adult minimum wage, according to a new study.
The GMB union said its research showed how urgent it is to stop having lower pay rates for younger workers.
The adult rate is £7.83 an hour, £7.38 for 21 to 24-year-olds and £5.90 for workers aged 18 to 21.
GMB is backing a Private Members Bill calling for age banding in the national minimum wage to be scrapped, which is set to be debated by MPs in the House of Commons on Friday.
Melanie Bartlett of the GMB said: "Hundreds of thousands of young people across the country are getting an unfair deal from this Government.
"Companies can currently pay under 25s up to 46% less for doing the same job as older, sometimes less experienced colleagues.
"This policy has little to do with ability - it discriminates on the basis of age and nothing else and shows this Government doesn't value the contribution young people make to our workplaces and economy.
"We have heard from countless young members how this discriminatory policy has a negative impact on their lives and traps young people in 'in-work poverty'.
"Many young workers have young families to look after and housing costs to pay, and cannot afford their living costs even with two adults working."
Up to a million workers under the age of 25 are losing as much as £3,513 a year because they are not entitled to the national living wage, according to the Young Women's Trust, a charity that supports young women on low pay.
Chief executive Dr Carole Easton said: "Young people are being paid less for the same work and are increasingly struggling to make ends meet. For young women, breaking out of low pay can be especially hard.
"We all need a basic amount of money to get by, no matter how old we are. The bus to work costs the same, whether you're 24 or 26. Gas and electricity costs the same. Rent doesn't cost any less in your early 20s.
"Politicians should support young people seeking to be financially independent by significantly increasing the apprentice minimum wage and changing the law to ensure under-25s are entitled to the same national living wage as everyone else. This would benefit businesses and the economy too."