Up to one in three workers are now in "precarious" employment such as jobs in the so-called gig economy or on zero-hours contracts, a new study shows.
Research by the GMB union found that "insecure" work was affecting the health of employees.
A survey of 1,000 workers in precarious jobs showed that three out of five suffered from stress and a similar number say the cost of living is increasing more than their pay.
Four out of five had previously been in permanent jobs, highlighting the changing nature of the workplace, said the GMB.
General secretary Tim Roache said millions of people were going to work not knowing what their hours will be.
"That is a sorry state of affairs in the 21st century and a product of government's failure to tackle bogus self-employment, the use of agency contracts as a business model and refusal to ban zero-hours contracts," he said at the union's annual conference in Plymouth.
"We hear a lot about employment figures but this research shows that the jobs market is based on a shaky foundation of insecure work where people are doing their best but are still not able to get on."