The number of workers categorised as self-employed has risen to its highest in 40 years, according to the Office for National Statistics (ONS).
Around 4.6m people work for themselves in the UK, or 15% of the total numbers employed.
That compares with 13% of the workforce at the start of the financial crisis in 2008.
However, the average income of self-employed workers has slumped by 22% over the same period, says the study.
Since January 2008, 1.1m extra people have found employment.
Of those, 732,000 - or two thirds of the total - categorise themselves as self-employed.
The ONS figures help set the overall fall in unemployment into context. Unemployment in the UK peaked at 8.4% in 2011, but has since fallen to 6.4%.
The TUC described the figures as "worrying", as the pay of self-employed workers is typically around half that of people in staff positions.
"The growth in self-employment is reducing people's pay, job security and retirement income - and is likely to be reducing the government's tax take too," said Frances O'Grady, the TUC general secretary.
But the figures could also be evidence that more people want to run their own businesses, with more control over the jobs they do.
"Many people aspire to be their own boss," said a government spokeswoman.
"Self-employment has been a growing part of the labour market for most of the last 30 years, which is why we continue to support budding entrepreneurs," she added.
The main reason for the rise in the total was that fewer people left self-employment during the period.
This trend could be accounted for by more people over the age of 65 continuing to work. The self-employment rate among this group has doubled over the last five years.
The ONS also said the economic downturn may have discouraged people from taking up staff positions, encouraging them to remain self-employed.
Typical jobs which have seen a big rise in self-employment include management consultancy, photographers and chartered accountants.
But the most popular self-employed sectors remain:
- construction and building: 167,000 people
- taxi drivers and chauffeurs: 166,000
- carpenters and joiners: 144,000