The true scale of unemployment in the UK could be as high as 6.3 million if a different counting measure was used, according to a new study.
The official jobless total increased to 2.68 million last month and is expected to rise again when new figures are published by the Office for National Statistics on Wednesday.
But the TUC said that under the American measure, which includes people in part-time jobs because they cannot find full-time work and recent redundancies, it would be more than twice the official total.
At 6.3 million, unemployment would be higher than at any point since the early 1990s.
Under-employment, which counts those doing temporary or part-time jobs because they cannot find permanent, full-time work, has risen to a record 1.9 million, according to the research.
The TUC said temporary jobs were better than unemployment, but added that they tended to be low paid, insecure and offer little or no career prospects.
The group of union leaders called on the Government to acknowledge the scale of the jobs "crisis" rather than repeat the "ill-informed" claim that there were plenty of jobs available.
TUC general secretary Brendan Barber said: "The headline unemployment figures are bad enough, but the true scale of joblessness is even worse.
"Our jobs crisis is not confined to those out of work. Nearly two million people are being forced to take low-paid, insecure, short hours jobs because of the lack of proper full-time employment. This means people are taking home much less pay, which is putting a real strain on family budgets.
"Unless we get people back into decent jobs and wages growing in line with prices again, we will not secure a sustainable economic recovery nor get the deficit down."
The report followed a study by the Chartered Institute of Personnel and Development which showed that job prospects are set to worsen in the coming months as firms make workers redundant.
Deputy Prime Minister Nick Clegg also gave further details about the Youth Contract on Monday, the Government's push to get more 18 to 24-year-olds into work.
But this week's figures are expected to show the eighth consecutive month of rising unemployment.
The Institute for Public Policy Research ( IPPR ) said it has been three years since unemployment broke through the two million mark.
There are 600,000 more people working part-time who say they want to work full-time, compared to three years ago, according to the think-tank.
Graeme Cooke, IPPR's associate director, called on the Government to guarantee everyone who has been unemployed for more than a year a job at the minimum wage in local government or the voluntary sector.
However, he added: "But with that right should come the responsibility to take that job or risk losing their benefits."
Mr Cooke said: "The longer someone is unemployed, the less likely they are to ever return to work.
"Being out of work for more than a year can have a scarring effect, making it harder to get a job as well as having a negative impact on one's health and well-being.
"This means that even when employment starts to pick up again, they will find it hard to compete with other jobseekers and could find themselves permanently shut out of the jobs market."
A spokeswoman from the Department for Work and Pensions said: "Tackling unemployment is a priority for ministers, which was why the Government acted quickly to replace the numerous failing back to work schemes with the Work Programme.
"It is the biggest single payment by results employment programme Great Britain has ever seen, with hundreds of organisations involved."
But shadow work and pensions secretary Liam Byrne said the TUC's analysis showed the "true scale of our country's unemployment emergency".
He added: "Unless ministers act soon a whole generation will be scarred by long term unemployment."
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