A survey of more than 2,000 workers earning £6.50 an hour or less showed that one-third cannot afford to shop where they work, and half did not know how much they would be paid from week to week.
The poll, for Unite, also revealed that most of those earning the minimum wage believed their employer could pay them more.
One-fifth of young minimum wage workers said they had been forced to turn to food banks in the past year.
Unite assistant general secretary Steve Turner said: "Government and business in this country are in danger of contriving a situation where hard work does not pay.
"Five million UK workers earn less than a living wage, consigned to an insecure income and increasingly shut out of the economy.
Many of these workers are employed in the shops and services that populate our high streets, yet they cannot afford to shop where they work."
"The crying shame is that low-wage work in this country is no longer the first rung on the employment ladder but actually the first step into a poverty pay trap."