A "staggering" 5.5 million workers could be on zero hours contracts, according to a study of 5,000 members of the Unite union.
A study found more than one in five Unite members were on zero hours contracts - five times higher than prior estimates - with half of respondents in the 16-30 category.
Unite general secretary Len McCluskey said the research suggests a "growing sub class" of insecure, low paid employment, with only one in seven wanting to remain on the contracts.
The results of the study by social survey company Mass1 came as employment rights will be debated on the first day of the TUC congress in Bournemouth today.
More top news
Cloudy skies for many this Bank Holiday Sunday.
The brother of British holidaymaker Steven Cook who vanished in Crete 10 years ago has his family are "sure he's alive".
Neurologist and author Dr Oliver Sacks CBE who wrote popular case histories like “The Man Who Mistook His Wife for a Hat", has died aged 82.