The number of care workers on zero-hours contracts has jumped from one in 10 of the sector’s workforce to one in seven in the past year, according to an analysis of official data.

The increasing use of zero-hours contracts is revealed as a trade union boss described the abuse of minimum wage laws in the care sector as “endemic”. Dave Prentis, general secretary of Unison, said hourly pay was often reduced to levels below the legal minimum because care workers who visited the homes of those needing help to live independently were paid only for time spent with each client. They receive no pay for time spent travelling between appointments. “Without this exploitation and the sheer dedication of the workforce, the social care system would collapse,” Prentis said.

Data from the Office for National Statistics showed that between April and June this year, about 113,000 of the 769,000 workers who provided at-home care for vulnerable people or were employed in care homes were on contracts with no guaranteed hours. At approximately one in seven, that total represents a substantial and rapid increase on 2015, when one in 10 care workers were on zero-hours deals.

The analysis is published as part of a Guardian series investigating the increasing number of people in precarious work in the UK. Earlier this week it was revealed that more than one in five workers, or 7.1 million people, now work in conditions where they could lose their jobs suddenly.

Prentis accused HMRC of failing in its duty to ensure that workers receive the minimum wage. “The abuse of minimum wage laws is endemic across the homecare sector because the non-payment of travel time drags already low wages below legal minimums,” he said. “HMRC is meant to ensure all employers comply with the law, but it is failing to do so.”

The union is particularly concerned about underpayment to workers who travel to people’s homes to prepare food, get them in and out of bed, and help with other tasks that allow them to live independently. These workers are often paid an hourly rate – but only for the time they are in people’s homes. Time spent travelling between appointments is not paid.

Workers who are paid less than legal minimums – £7.20 a hour for those aged over 25 – can take their complaints to Acas, which will refer them to HMRC to investigate.

More than 130 care firms are currently under the scrutiny of HMRC; in the two years to 31 March 2016, it found employees were owed almost £1m in missed pay as a result of breaches of minimum wage law.

The Labour MP Paul Blomfield, who has raised the issue of care workers’ wages in parliament, said he believed the practice of not paying workers for the time they needed to travel between visits was widespread.

Blomfield said he knew of one resident in his Sheffield constituency who was “crisscrossing” the city to visit clients but not paid for her journey time: “She was working for eight hours or so, but being paid only for four hours.”

Prentis said the government, HMRC and councils all knew that non-payment for travel time was commonplace. He called on the government to “ensure that pay laws are properly enforced and give social care the resources it needs in this month’s autumn statement”.

Blomfield also criticised HMRC for not having a formal process to revisit employers who had flouted minimum wage laws and for dealing only with the complaint of the employee who had raised a concern about their wages – rather than checking the payments received by all employees of offending firms.

In a letter to Jane Ellison, financial secretary to the Treasury, the MP says: “From the 36 social-care employers that have been named and shamed to date, 21 of these employers have had just one worker identified as being owed arrears.”

He added: “There is a likelihood that other care workers at these providers may not have been paid what they are historically owed, and so checks on their position would seem appropriate.”

Haringey council and the care agency Sevacare are currently facing a case brought by 17 employees who say they were earning as little as £3.27 an hour for providing live-in care or visiting people’s homes. Sevacare said it paid the staff “an average hourly rate, that is at least the minimum wage.

Earlier this year Sevacare was involved in another case where a care worker in Manchester was awarded more than £3,000 for unpaid hours while making house visits.

Judith Montgomery, the care worker who brought that case, said she regularly spent longer than the scheduled appointments and did a 50-hour week for 30 hours’ pay. “In a 15-minute appointment you would be expected to get somebody up, get them washed and dressed, give them their medication and fill in the paperwork,” she said. “You could be there for 25 or 30 minutes but you would only be getting paid for 15.”

Unison’s Prentis said cases like Montgomery’s were rare, but only because it was so often difficult to decipher the information on payslips. “Cases against employers are rare because payslips are so difficult to understand, and the widespread use of zero-hours means employees are reluctant to complain, for fear their hours may be taken away.”

Blomfield has also met with the government as part of a move to make payslips easier to read. In some cases payment advice runs over many pages and can be almost impenetrable. On one typical payslip seen by the Guardian, a worker appeared to be listed making two visits at the same time.

A number of care workers told the Guardian that their hours – and income – could fluctuate markedly and reported they were afraid of taking time off, even to deal with family crises, in case they upset their employers and lost shifts as a result.

Citizens Advice said it had heard from many workers in the sector who had problems linked to the insecurity of their jobs. Gillian Guy, chief executive of the charity, said: “Not knowing what hours you’re working from one week to the next, or how much money you will lose to travel costs, can make it very difficult to manage household finances. It can also cause problems with other aspects of money management, such as whether you can get a mortgage or even commit to a mobile phone contract.”

According to the UK Home Care Association, which represents care providers, councils need to pay agencies £16.70 an hour to cover the cost of paying workers and provide enough profit to keep the businesses sustainable. The average paid by councils which provided data was only £14.58.

Blomfield said: “We can’t allow care workers to be bearing the brunt for decisions made way above their heads about how much we are willing to pay for social care.”

A HMRC spokesman said: “All businesses, irrespective of their size or business sector, are responsible for paying the correct minimum wage to their staff. HMRC continues to crack down on employers who ignore the law. At the start of the year HMRC had 130 open investigations into care providers as a result of complaints made by employees and as a result of our targeted enforcement campaign, ensuring that care workers receive the wages they are entitled to. As part of this we are taking targeted action against some of the biggest social care providers.

“If anyone is concerned that they are not receiving the minimum wage, they can contact ACAS, in confidence, on 0300 123 1100. The Acas helpline is open Monday, Wednesday, Thursday and Friday 8am to 8pm, Tuesday 8am to 6pm, and Saturday 9am to 1pm. Cases can be dealt with anonymously, with no risk of employees being identified.”