Samantha Nelson doesn't dawdle. She's up at 5.30 every morning and quickly in her car for the seven-mile journey that takes her to a gruelling day of work that often ends at around 11pm.

A carer who has washed and fed elderly people in her town for the past 14 years with the same privately owned company, Nelson has been rewarded with a precarious existence: a zero-hours contract that pays below the minimum wage.

By law, no one over the age of 21 should be paid less than £6.19 an hour, but Nelson's employers hit on a ruse – to pay only for "care minutes" with a client. The result is that Nelson is not paid for the time taken to drive 50 miles a day and her hourly pay rate works out at more like £5.

Essentially on standby, she can be called up to whizz between houses in shifts as short as 15 minutes. Today, seven in 10 care workers cope with such insecurity, up from one in three five years ago.

The care industry, which employs more than a million people, has made a name for offering a personalised service. However, it relies on Orwellian oversight of its workforce.

Care workers are routinely tagged with tracking devices to ensure that the time, location and duration of visits are logged. Too short a stay means no pay. "For a 30-minute visit you need to be there at least 23 minutes and arrive within seven minutes of the scheduled time. Even if you finish up early and you are late for the next appointment, you can't leave. You won't get paid. It's Big Brother, except it's no joke," said Nelson.

Nelson, who asked to use a pseudonym for this article, said: "The owner knows what she's doing. What's not in your pay packet is in the owner's. There are 200 women here. If you bring it up you will suddenly have your number of hours cut. One girl went down to eight hours a week when she asked about it. You cannot live on that. So we all keep quiet."

The scale of such law-breaking on pay is breathtaking in an industry that supports millions of vulnerable, elderly people. A study in 2011 by King's College London's social care workforce research unit estimates that there are between 150,000 and 220,000 care workers are being paid below the minimum wage.

These transgressions are not easy to prove, not least because of the labyrinthine nature of wage slips detailing every minute paid.

Compared with the issue of unpaid interns – a matter concerning 100,000 young people, mostly graduates desperate to get a foot on the career ladder – care workers are a forgotten workforce. Mainly older women, often migrants, lightly skilled and with no union clout, their cause has attracted few powerful friends.

The result is that in the past tax year, Her Majesty's Revenue and Customs investigated 40 cases of companies hiring unpaid interns and took action against nine, leading to fines and repayment of salaries to 167 individuals of a total of £192,808. By contrast, in the domiciliary care sector, companies were fined and forced to repay wages of just £23,655 to 237 workers.

What surprised many experts was that when the vexed issue of social care preoccupied the political class in early 2011, there was little debate over an issue upon which the whole care system rests: whether the quality of the workforce was affected by absurdly low wages.

Gavin Kelly – chief executive of the Resolution Foundation, a thinktank that looks at low pay – has been concerned for months that the issue has failed to gain political traction.

He said it was disgraceful that "in a workforce that is often overlooked, with little public profile, too many employers have been getting away with breaking the law for far too long and HMRC, councils and Whitehall have failed to respond. If this was happening elsewhere in the public service, I expect there would be much more of a public outcry."

There are signs that politicians are getting the message. Labour-run Islington council in north London says that from March next year, only companies paying care workers the London living wage – worth £8.55 an hour – will get contracts from its £6m care budget. The council leader, Catherine West, said she had been concerned that poverty wages "would drive employees out of reach of Londoners".

In Westminster, too, some ministers are waking up to the issue. The Lib Dem health minister, Norman Lamb, said he had asked his colleagues in the Department for Business, Innovation and Skills to name and shame those firms not paying staff the minimum wage.

Lamb, who on Thursdaychairs a summit with care providers, councils and trade unions , aims to "produce a piece a work showing you can improve workers' conditions. After all, what sort of care do you think we are providing some of the most vulnerable people in society at these rates?"

Some are not waiting to find out. Debra Claridge (pictured above), who had worked in the petrochemical industry for 25 years, "ended up in care work" around the West Midlands after her family firm got into difficulties in 2011.

However, she was horrified at the way care workers were treated – intensely monitored at work, then bombarded with calls from managers when they were supposedly off duty in order to provide cover for colleagues. She claims she was also illegally paid below the minimum wage and that when she complained, she was marked out as a troublemaker.

"I had worked in other industries and I could find out what the law was. I was disgusted by my employers and their attitude to my pay," said Claridge, who left the industry last November – less than 18 months after she started – to work in retail.

Rather than lie low, she took her case to HMRC, pointing out that in a typical week she worked almost 30 hours but was paid for less than 24 of those – a loss of earnings of about £33 a week. The result has been a one-woman campaign to get the issue taken seriously. After six months of letter-writing to MPs and ministers, she won a partial victory last week when HMRC agreed that the company's explanation for underpayment does not "cover the working period from a national minimum wage point of view".

Claridge says the fight is not just for her, but for the almost 100 employees at her old care firm. "If all the care workers in my previous company were underpaid, then the loss to us was about £195,000. "