Almost 37,000 people in Scotland may have been paid below the national minimum wage last year, according to Citizens Advice Scotland.
The charity said analysis of HMRC data for the UK suggests 36,876 workers were potentially paid less than the statutory requirement in Scotland in 2018.
CAS wants national minimum wage breaches to be investigated, saying the figure is “just the tip of the iceberg”, with many more underpaid employees unidentified by the official figures.
The charity’s social justice spokeswoman Mhoraig Green said: “Our advisers gave out almost 12,000 pieces of advice relating to pay and entitlements at work last year, and we regularly see cases of employers paying illegally low rates or refusing to pay holiday pay – which all workers are entitled to.
“Particularly common are cases involving non-UK nationals and workers who are paid ‘cash in hand’ or not given written contracts or payslips.
“The figures we publish today are simply unacceptable. Employers need to understand that paying the statutory minimum wage rate for their workers is not optional. It is the law.”
She added: “This should act as a wake-up call to our new Government to ensure that 2020 is the year the minimum wage must become a legal reality.”