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
Judge Mindy Glazer was not expecting to be reunited with "the nicest kid in middle school" when Arthur Booth appeared in her court.
The WikiLeaks founder has been living in the Ecuadorian embassy in London for three years but claims his life is in danger.
Small shops in some parts of Greece are in danger of running out of coins and small change as cash reserves in the country begin to run low.