Staff of fast-food outlets and bars are the most likely to fall prey to employer abuse of flexible working hours, claims a new report.
The National Association of Citizens Advice Bureaux (Nacab) says there is "intolerable insecurity" among workers because of a one-sided flexible labour market.
A growing practice, says the report, is to give workers "zero-hours" contracts that give no guarantee of any work or pay each week.
The problem is rife in the hospitality industry, particularly in bars and fast-food outlets, according to Mick Graham, public services national secretary for the GMB union.
"Zero-hours contracts should be outlawed, they tarnish the whole industry," he said. "In fast-food places youngsters are often paid only when they are actually serving," he added.
Burger King was forced to pay out £106,000 in compensation to 900 workers last year after staff were told to clock off when business was slack. This referred to making shift workers take unpaid breaks rather than the practice of zero-hours contracts, which is legal.
Nacab wants zero-hours contracts to be outlawed.