The figures show the rise has mainly been since 2011: of the 367,000 increase in self-employment, 219,000 was between 2011 and 2012.
By contrast the number of employees, which fell 434,000 between 2008 and 2012, dropped mainly at the beginning of the period, with a drop of 600,000 between 2008 and 2009, with the figures showing a partial recovery since 2010.
The increase in self-employment took place across all parts of the UK, with the exception of Northern Ireland, where the number of self-employed workers decreased, the report revealed.
It also found there was an increase of 431,000 over that period in the number of self-employed people who worked on their own or with a partner, but a drop of 66,000 in the number of self-employed workers who had employees working for them.
Today's report shows that self-employed people work longer hours than employees - on average 38 hours a week compared with 36 for employees.
Self-employed workers tend to be older than employees and are more likely to be male - in 2012 the average age of the 4.2 million self-employed was 47, and 70% of them were men, while the average age of the 25 million employees was 40 and only 51% of them were men.
The four most common occupations for self-employment were taxi or cab drivers (166,000), 'other construction trades' (161,000), carpenters and joiners (140,000) and farmers (123,000).
The report revealed that 58% of self-employed people used their home for work purposes to some extent, either working there (15%), using it as a base (38%) or working on the same grounds or building as their home (5%).
Xenios Thrasyvoulou, CEO and founder of online freelance site PeoplePerHour, said: "What began as a trickle of people starting to work for themselves has turned into a tide.
"When the downturn first hit, many people who had lost their jobs became self-employed through necessity.
"But the surge in self-employment between 2011 and 2012 suggests that more of us are making the change out of preference rather than panic.
"Most striking of all is the age profile of the newly self-employed - the vast majority are over 50."
Thrasyvoulou added: "Baby-boomers are leading the self-employment revolution - and this echoes our own data which shows an 88% increase in baby-boomers starting their own business or going self-employed in the past year."
Chris Futcher, CEO The Pulse Umbrella Group, said: "At the beginning of the economic downturn people were initially hopeful that things would be back to normal in a relatively short time.
"However, as the economy continued to suffer, and more and more people found themselves out of work with limited employment options, this likely prompted the increase in people deciding to become self-employed.
"Becoming self-employed does bring with it a number of benefits such as being your own boss, however it is important to keep close tabs on your finances so as not to get any nasty surprises at the end of the tax year.
"Also, 2011 saw the introduction of the AWR. As a result, many suppliers of temporary workers in some sectors misguidedly advised workers to become self-employed as a possible avoidance measure. It is extremely unlikely that these workers would pass the self-employment test.
"As an employer, if you are using someone who is self-employed, it is up to you to ensure that they pass the self-employment test.
"The test is quite complex and includes control, risk, supply of materials and equipment and usual conditions in the particular industry."
