Seventeen care workers are alleging failure to be paid the minimum wage in the sector's biggest ever legal claim.
Payslips appear to show contractor Sevacare had some staff in Haringey, north London, on a rate of £3.27 an hour - less than half the then minimum.
Their union Unison says an employment tribunal will examine some of the worst breaches of pay rules it has ever seen.
Sevacare says it pays above minimum wage but that the workers feel hours it deemed off-duty should also be covered.
The company has contracts with a number of local authorities across England, providing care and support to 9,600 people each week, but no longer has a contract with Haringey.
The workers involved in the case - sixteen women and one man - were employed by Sevacare to look after people in the borough over many years up to 2016.
According to Unison, the £3.27 rate was being paid to a number of women who acted as "live-in" care workers at the time the national minimum was £7.20 an hour.
It says they stayed, for seven days at a time, in the home of an elderly woman with severe dementia.
The carers say they were on duty 24 hours a day - they slept on a bed in the same room as the woman, and would often have to tend to her needs through the night.
One carer likens the experience to being ''in prison'', saying they were not allowed to leave the house all week.
All of the care workers were on zero-hours contracts and Unison says the women felt they could not turn down work because they feared being penalised by the company.
Unison's general secretary Dave Prentis said: "Unfortunately this sorry state of affairs is not unique to Haringey. Up and down the UK, the experience of other home care workers is similar."
He said that the workers were paying the price of the government's "penny-pinching approach", while the companies employing them were "coining it in".
Sevacare disputes the idea that the women had been working 24 hours a day.
It says the "live-in" care workers' hours were covered by a "daily average agreement" which saw them paid for 10 hours. The care workers were actually receiving £550 for a seven-day week, it says, which is the equivalent of £7.85 an hour.
"Sevacare submits that it pays, and had paid to these staff, an average hourly rate that is at least the national minimum wage over any pay reference period," the company added.
The tribunal also involves former Sevacare staff in Haringey who claim their pay rates fell below the minimum wage because they were not paid for travelling between visits to clients.
Haringey Council, which was responsible for commissioning the care, is also being sued at the employment tribunal for failing to ensure the workers were paid properly.
The council says it ended its relationship with Sevacare in April this year after previously raising ''serious concerns'' with the Care Quality Commission.
However, the BBC has seen payslips that suggest some care workers currently working in the borough are still not being paid the legal minimum.
The hourly rate printed on recent pay documents is well below the national living wage of £7.20 an hour, which came into effect in April and raised the minimum wage from £6.70 hour for employees over the age of 25.
A spokesman for Haringey said the council contractually requires providers to obey minimum wage law, including remuneration for travel time.
"Our rates allow for providers to pay both the national minimum wage and the national living wage, and we will be writing to them all to remind them of their Care Act obligations, stating clearly that no breaches will be tolerated," he said.
'Treated like slaves'
It is a joy. It is a pleasure. That is how Juliet Henry, one of those taking action against Sevacare, describes her job of caring for the elderly and the vulnerable.
She says she went into caring eight years ago because it made her feel good to give something back to people who are less fortunate than her.
She speaks movingly of the bond of friendship and trust she has built with her clients.
''They're so glad to hear that key in the door. So glad you're there to look after them,'' she says.
Ms Henry visited all of her clients by bus. But there were some days, she says, when she spent more time trying to get to her clients than caring for them - she could spend seven hours travelling each day and not be paid for it.
Florence Wambulu is very fed up.
For years, she says, she worked seven days a week as a care worker in order to make sure she could pay her bills and look after her family. But Ms Wambulu says she was overworked, and her family grew concerned for her health.
''They have to treat us like human beings, not just someone who is there to make money for them," she added. "We were working like slaves.''
She says she tried to improve the working conditions by talking to Sevacare, but says: "If you tried to take a day off, they threatened to take all the hours from you.''
'Low pay scandal'
And last year Her Majesty's Revenue and Customs announced it would investigate whether the big six care providers were paying people the minimum wage.
While stressing it followed its minimum wage obligations, Sevacare acknowledged the tribunal case did highlight areas relevant to employees across the sector, adding: "The fundamental underlying issue undoubtedly is the inadequate funding of social care for the elderly by government."
For its part, Unison says there is a "scandal" of low pay throughout the sector.
Mr Prentis said: "The government, local councils and the care companies all know that social care is in a dire state, that there's not enough money to pay for the care that's needed."
Urging the government to take action, he said ministers must get tougher with enforcing the law so that firms cannot cheat their staff.