The number of self-employed workers has jumped by 367,000 since the start of the recession in 2008, official figures have shown.
The increase - to 4.2 million - has happened mainly since 2011, with a rise of 219,000 in the year to 2012, said the Office for National Statistics.
All parts of the UK experienced a rise except Northern Ireland, where the number of self-employed workers fell.
The study also revealed that the self-employed work longer hours, tend to be older and are more likely to be male, than other employees.
A total of 70% of self-employed workers are male, with an average age of 47, compared with an average age of 40 for the UK's 25 million employees.
The most popular occupations for self-employed workers are taxi driver, construction trades, carpenters, joiners and farmers.
About 58% of self-employed people use their home for work, with most living in London and the South West.
The lowest proportions of self-employed people were in the North East, Scotland and Yorkshire and the Humber.
TUC general secretary Frances O'Grady said: "Self-employment is normally a very small part of the workforce, so the fact that it's been outstripping employee job growth shows that the UK labour market is far weaker than headlines suggest.
"The recent rise in job levels is being driven by self-employed, part-time and temporary jobs, rather than the full-time, permanent work that many people want and need."