The number of people doing gig economy work has doubled in the last three years, with young people most likely to be doing this type of flexible, insecure work, the TUC says.
The trades union body says the majority of people using apps such as Uber or Handy to find work have other roles.
This shows that “working people are battling to making ends meet”, said TUC chief Frances O’Grady.
One in 10 working age adults find work via apps or websites, the TUC said.
That compares with about one in 20 in 2016.
The data comes from a survey carried out by the University of Hertfordshire and Ipsos Mori.
Ms O’Grady said: “Huge numbers are being forced to take on casual and insecure platform work – often on top of other jobs.
“But as we’ve seen with Uber, too often these workers are denied their rights and are treated like disposable labour.”
The survey of 2,235 UK residents found that young people were most likely to be using apps to find work and accessed via laptops and smartphones – known as “platform workers”,
The survey found that nearly two-thirds of workers using apps to find work at least once a week were aged between 16 and 34.
One in seven (15%) of workers had been involved in platform work in some way, which the TUC said equated to 7.5 million people.
Changing world of work
Ursula Huws, professor of labour and globalisation at the University of Hertfordshire, said the work being sought was not just taxi driving and food delivery.
“They’re only a small proportion of gig workers. They’re outnumbered by an invisible army of people working remotely on their computers or smartphones or providing services in other people’s homes,” she said.
Six in 10 of the respondents to the survey admitted buying services in this way at some point.
The TUC said the survey showed that it was time for all workers to get basic rights such as the minimum wage and holiday.
“The world of work is changing fast and working people don’t have the protection they need,” Ms O’Grady said.
At the end of last year, the government said it was introducing measures to give better protection to workers on zero-hour contracts, agency employees or gig economy workers.
Staff would have to be told details of their rights from their first day in a job, including eligibility for paid and sick leave, and given the right to require more predictable hours.
In February 2018, the Department for Business, Energy and Industrial Strategy published research showing that 4.4% of the population in Great Britain had worked in the gig economy in the last 12 months.