One in 10 employees wants to work more hours every week, but a similar number would like to put in fewer hours for less pay, a new report has shown.
Around three million people are classed as under-employed, wanting to work an extra 11 hours a week on average, said the Office for National Statistics (ONS).
One in five part-timers is under-employed compared with one in 20 in a full-time job.
The North East has the highest percentage of under-employed workers in Britain.
The study also revealed that just under 10% of workers want to work fewer hours and so are classed as over-employed.
Dr John Philpott, director of The Jobs Economist, said: "As expected, the number of under-employed workers fell by 116,000 in the year to Q2 2014 but the figure remains staggeringly high, marking the sixth successive year in which the under-employment rate has been above 9.5%.
"Such a prolonged period of mass under-employment demonstrates the extent to which the very good headline employment and unemployment figures of recent years mask a substantial underlying shortage of work, the persistence of which takes some gloss off the UK's supposed 'jobs miracle'.
"With almost three million people under-employed alongside still almost two million unemployed the pain of work shortage and associated pay weakness is likely to continue well into 2015."
TUC general secretary Frances O'Grady said: "Under-employment is still much higher than it was before the recession, which is a clear sign that the UK isn't creating enough of the right kind of jobs.
"There is a big shortfall in the supply of full-time employee jobs, and that's making it harder for families to earn enough for a decent standard of living.
"It makes no sense that there are so many people wanting more work, while others complain of being overworked and want fewer hours. The Government should strengthen flexible working rights, which could help solve both problems."