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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.
The Government has hit back at TUC claims that the UK remains in a job crisis, with a true scale of unemployment that far exceeds the official figures.
The TUC has said the UK is "still in the midst of a job crisis" despite unemployment falling in recent months.
Mr O'Grady said it was important that official figures "better reflect the true nature of the jobs market".
The TUC Congress opens in Bournemouth today with a warning that unemployment could be as high as 4.75 million.
A TUC report published ahead of the south coast meeting said wider measures of joblessness should be incorporated into the total, including the economically inactive.
As well as the 2.5 million official jobless figure, a further 2.26 million people want a job but are not classified as unemployed, the TUC said.