The number of people on zero-hours contracts, doing temporary or agency work, in low-paid self-employment or other forms of insecure work could soon hit 3.5 million, according to the TUC.
It estimates that 290,000 more people could be trapped in insecure work by 2022 if current trends continue. That number is equivalent, the TUC says, to 13 extra Sports Directs or the entire working population of Sheffield.
Research previously undertaken by the union body found that workers on insecure zero-hours contracts earn a third less per hour than the average worker.
Adding up the income tax and national insurance contributions (NICs) not paid by such workers, and the benefits and tax credits paid to them, the TUC estimates that insecure work costs the Treasury £4 billion a year.
In order to tackle the problem, the TUC wants a ban on zero-hours contracts, with people working regular hours having the right to a guaranteed-hours contract.
People on variable-hours contracts should be paid overtime for any hours they work outside their contracts, it goes on, and all workers should have a right to a written statement of terms, conditions and working hours, from the first day of their employment.
In addition, the TUC is calling for everyone at work to get the same rights as an employee, unless the employer can show that they are genuinely self-employed, while agency workers should be entitled to the going rate for a job, on an equal basis with directly-employed workers.
TUC General Secretary Frances O’Grady said: "The next Government will need to tackle this problem head on. Every party manifesto must have real commitments to crack down on zero-hours contracts and bogus self-employment. And agency workers should always get the going rate for the job."