The Government is being pressed to spread the national living wage to all workers and to raise rates to match the high cost of living.
Unison said the prevailing hourly rate of £7.50 for over 25-year-olds should be increased to the voluntary living pay of £9.75 in London and £8.45 in the rest of the country.
In its letter to the Low Pay Commission, Unison said many public sector labourers were striving because of “poverty” pay.
General secretary Dave Prentis said: “Poverty pay should be outlawed in the 21st century. Yet employers are still cheating staff out of their rightful earnings.
“It’s also wrong that young workers doing exactly the same job as older colleagues are paid less per hour just because of their age.
“Everyone benefits if staff are paid a decent wage. They deserve an hourly rate that reflects the real cost of living for them and their families.”
Unison calculated that over 160,000 workers in the local state, the NHS, higher and further education were given below the £8.45 voluntary living wage over the past year.
A Business Department spokesman said: “The Government continues to use age-related rates to protect the employment prospects of younger workers.
“The wages of younger workers will continue to be underpinned by the core National Minimum Wage at the highest possible level.
“The Government also sets minimum pay thresholds only, and we recognise and commend those employers who set higher rates of pay for younger workers where they can afford to do so.”