Government plans to review zero hours contracts are "inadequate" and need to be replaced by a more in-depth investigation, according to a research group.
The Work Foundation said the use of zero hours contracts, under which people do not know if they are working from one week to the next, varies between sectors.
Almost one in five hospitality firms use the contracts, but they are rare in retailing, a study of the Labour Force Survey found.
Business Secretary Vince Cable has announced a review following reports that growing numbers of workers are being employed on zero hours contracts.
The Office for National Statistics said 250,000 workers were on zero hours contracts, but the Chartered Institute of personnel and Development revealed it believed the figure was nearer a million.
Ian Brinkley of The Work Foundation said there "confusion" over the numbers of workers involved, adding: " There are vast numbers of workers who are unaware they are on zero hours contracts.
"We still don't know how many have taken them by choice and how many out of necessity. Nor do we yet understand the true reasons why employers are making more use of them.
"The investigation announced by Vince Cable is inadequate.
"A fuller investigation would enable accurate analysis and such data would help devise effective policy measures and map out best employer practice to protect workers most vulnerable to potential abuse."