Under-employment in the UK has jumped by 93,000 over the last two years, according to the Trades Union Congress (TUC).
The organisation, which analysed the Office for National Statistics' Labour Force Survey, said this was despite unemployment falling by more than 400,000 since early 2012.
The TUC also said that the current level of under-employment is more than a million higher than it was before the recession – an increase of 46%.
"With more jobs being created, you'd expect that under-employment would be on the wane too," said Frances O'Grady, the general secretary of the TUC.
"But sadly with part-time, temporary, low-paid jobs often the only work that people can get, under-employment remains stubbornly high and is still rising."
Self-employed, but under-employed, rise
The research also revealed that the fastest rise in under-employment over the past two years has been among self-employed workers.
The TUC explained there has been a 7% (36,000) increase in the number of self-employed people who class themselves as under-employed, compared to a 2% rise (57,000) for employees.
The study also found that since the government came to power in 2010, under-employment has increased by nearly half a million (432,000) and shows little sign of slowing.
Northern Ireland had seen the sharpest increase in under-employment since the last election with a 37% rise, followed by the East of England (up 25%), Wales (up 21%) and the North West (up 18%).
The TUC also said that under-employment had also gone up "significantly" in the North East (16%), South East (up 14%), East Midlands (up 11%), and the South West (up 10%) since 2010.
'Misleading' TUC figures
"David Cameron's failure to tackle under-employment has left millions of people stuck in part-time jobs unable to get the hours of work they want," said Rachel Reeves MP, Labour's shadow work and pensions secretary.
"Labour is determined to control the cost of social security by getting people back to work and making work pay."
But the Department for Work and Pensions claimed that the TUC's figures were "highly misleading".
"Independent statistics show that there are more than 100,000 fewer people who say they are under-employed compared to a year ago, and that full-time jobs account for more than three-quarters of the rise in employment since 2010," said a spokesperson for the DWP.
"The proportion of part-time workers wanting a full-time job has just seen the biggest annual fall in over two decades.
"The overwhelming majority of those working part-time do so because it suits them. For example, students or those who want to work around caring or parenting responsibilities."