The number of workers on zero-hours contracts could be one million - four times as high as official estimates, according to new research. Such workers are on call to work when needed by bosses but employers do not have guarantee any hours.
Dave Prentis, general secretary of the Unison union, says zero hours contracts favour employers and make it harder for staff to complain.
He said: "The vast majority of workers are only on these contracts because they have no choice. They may give flexibility to a few, but the balance of power favours the employers and makes it hard for workers to complain.
"Not knowing from week-to-week what money you have coming in to buy food and pay your bills is extremely nerve-wracking. Having your working hours varied at short notice is also stressful and it makes planning, childcare arrangements and budgeting hard.
He added that zero hour contracts also call into question unemployment figures, which, he says, do not include those on zero hours who may not have any work.
The government is looking into zero hours contracts as new research says the number of people working such contracts could be one million.
Opinion varies dramatically among both workers and employees when it comes to zero hours contracts, as ITV News viewers demonstrate.