Official figures have highlighted a steep rise in the number of people wanting to work more hours - sparking a bitter debate on the Government's economic strategy.
The Office for National Statistics (ONS) found there had been an increase of one million people classed as under-employed since the financial crisis of 2008, taking the total beyond three million.
The three million figure included almost two million part-time workers, many of whom wanted a full-time job.
According to the ONS, the occupations with the highest number of under-employed workers included cleaners, caterers and labourers.
The average under-employed person was found to have earned £7.49 an hour, more than £3 less than someone who was not under-employed.
More than one in five of workers aged 16-24 were under-employed this year, compared with 10% of those aged 35-49, the ONS said, while the highest under-employment rates were in the East Midlands, Yorkshire and Humber, the North East and the South West where more than 10% of workers wanted to work more hours.
Yorkshire and the Humber saw the highest rate of increase.
The statistics were released against a recent backdrop of falling overall unemployment but they were seized upon as evidence that the Government's strategy was failing.
"Under-employment statistics expose this claim as a sham. "No wonder our economic growth has faltered - more than three million people are under-employed, many of them are stuck in part-time work but want full-time hours.
"Growing under-employment is masking broader economic problems and holding back the recovery, " she concluded.
Dr John Philpott, director of The Jobs Economist, said: "The extent of underemployment identified in these official figures highlights the degree to which the headline unemployment count understates the current shortage of work in the UK economy."
But John Salt of totaljobs.com, said: "Under-employment is the reason that we have managed to stave off the disastrous unemployment figures some had feared given the parlous state of the economy, but the numbers don't indicate any strength."
A Department for Work and Pensions spokesman responded: "More than half of the 700,000 extra people in work since 2010 are working full-time and we have recently seen record numbers of people in employment."