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.