The number of people in part-time jobs because they cannot find full-time work has increased by a million since just before the recession, according to a new study.
The TUC said there were now 3.3 million "under- employed" people in the UK, up by 42% since early 2008.
More than one in 10 workers are not doing enough hours, rising to one in eight among women.
Under-employment is worse in low skilled jobs, where one in five workers want to put in more hours, and in sales and customer service jobs, the research found. Teaching, nursing, legal and skilled business jobs have also seen an increasing number of employees working fewer hours than they want to, said the TUC.
Young people are almost twice as likely to be under-employed as any other age group, illustrating the depth of the youth jobs "crisis", said the report. Northern Ireland, East Midlands and the North West have seen the biggest rises in under-employment.
TUC general secretary Brendan Barber said: "A million people have lost their jobs since the eve of the recession in 2008, but this tragic figure only tells half the story. A further million people are now trapped in jobs that don't have enough hours to provide the income they need to get by.
"Young people, women and low skilled workers are bearing the brunt of our under-employment crisis. It is alarming just how few young people today are able to find a job working enough hours. This is a criminal waste of the talent and skills they have - all because of a crisis they didn't cause."
He added: "What the country needs is an economic strategy that puts people's futures ahead of self-defeating austerity. Cuts in infrastructure spending must be reversed and growing industries need more Government support. We also need banks to start lending again, so that businesses can grow and create jobs."
Employment Minister Chris Grayling said: "Being in some form of employment is better than being out of work. It is important that people are taking those first steps into employment through part-time work or jobs in different sectors as it provides vital experience and skills that employers will look for when the economy improves.
"Over the past few months the rise in full time work has outstripped that of part time and unemployment has continued to fall."