Employees on zero hours contracts are being paid 40% less per hour than other employees.
Earlier this month Management in Practice revealed that over a quarter of healthcare employers (27%) use contracts with no guaranteed shifts or work patterns.
Figures released by the Labour Party show that people on zero hours contracts earn £9 per hour on average, compared with £15 per hour for other employees.
As many as one million people in the UK are employed using the contracts, four times as many as the most recent Office of National Statistics figure.
Shadow Business Secretary Chuka Umunna said: ““New evidence highlights that there could be hundreds of thousands more people on zero-hours contracts than previously thought.
"That’s hundreds of thousands of people in insecure work earning far less than average pay. Flexibility works for some, but the danger today is that too often insecurity at work becomes the norm.”