A firm supplying hundreds of home carers has been accused of failing to pay workers for travel time between appointments, meaning they earn less than the minimum wage, a Guardian investigation can reveal.

Whistleblowers allege that Nationwide Care Services, which has eight branches across the Midlands employing more than 400 staff, does not pay its employees for the time taken to get to and from vulnerable people’s homes to provide care. Carers are also pressured to cut short appointments with elderly people to meet “impossible” rota schedules, the whistleblowers claim.

The allegations, which have been vigorously denied by the company, raise concerns over whether this means carers end up being paid less than the legal minimum.

It is understood that many of those working for the firm, which is contracted to provide care paid for by a series of councils as well as private clients, are employed on zero-hours contracts.

It is alleged that some office staff are doctoring logs to make it look as if carers are spending more time in people’s homes than they actually are. “As a result, the people suffer a poor standard of care, which is funded by me and you in the taxes we pay,” one whistleblower said.

It is understood that HMRC is investigating Nationwide Care Services over pay.

After being approached for comment, the firm’s director, Hamait Ali, posted a video on Facebook claiming the allegations against the firm were “totally untrue” and are made by “disgruntled ex-employees”.

Ali told the Guardian that the allegations were a personal attack intended to cause harm to the company’s reputation. He said that the company had never had any formal notice of a wage dispute.

The allegations have surfaced as a leading employment lawyer claimed HMRC is underusing its enforcement powers to sanction companies breaking minimum wage laws, with just seven prosecutions since 2010. According to the government, “travelling in connection with work, including travelling from one work assignment to another” counts as working time.

But one source the Guardian spoke to claimed tha ta carer at Nationwide Care Services was expected to attend nearly 20 appointments in a day, including four and a half hours’ travelling time taking buses and walking – yet they were paid for just nine hours’ work.

This would mean the worker, who was paid £8.92 an hour including a £1 bonus, was paid an average of £5.94 an hour for a shift lasting nearly 14 hours. The minimum wage for those aged 25 and over was £7.83 until April, when it rose to to £8.21.

The firm says on its website that it is “dedicated to providing consistently safe, high-quality, reliable care and support to people”. However, a judge criticised the company last month for its training after one of its carers left a dementia sufferer lying on the ground for two days with a slipper as a pillow.

The shadow minister for social care, Barbara Keeley, said: “The law is clear: time spent travelling between appointments must be paid at least the minimum wage. For any care provider to fail to pay care staff for travelling between appointments is unacceptable, and HMRC must take enforcement action to make sure that care staff are paid as they should be.”

Eileen Chubb, founder of Compassion in Care, a charity that campaigns on behalf of whistleblowers in the industry, added: “If you have a demoralised workforce who aren’t being paid properly, the knock-on effect is that the elderly people that are supposed to being cared for won’t be looked after properly. Vulnerable people deserve care that has been paid for.”

Whistleblowers also alleged that the company relies on inexperienced staff who have to complete gruelling shifts of up to 14 hours a day. “I usually woke up at 5am and ended up back at home at 1am. It was a nightmare,” one ex-carer said.

“They paid only for the time spent in the people’s houses. Some of the rotas were impossible ... I was usually working between 12 and 14 hours and I was being paid, depending how lucky I was, seven, eight, sometimes maybe 10 hours. But usually it was just half the time. If I worked 13 hours, I would be paid around half the time.”

Another ex-employee at the firm added: “The carers don’t last long in the role – long working hours, no travel time, seven days a week. The rotas are unrealistic, which means the care staff will have to cut calls in order to get to the next person on time.

“There is no way they can deliver the actual duration of the care call.”

Rakesh Patel, head of employment rights strategy at Thompsons Solicitors, said: “The firm appears to be in breach of the National Minimum Wage Act by failing to pay workers for travel time between assignments.”

Patel added: “The use of its enforcement powers has been pitifully underused by HMRC. The government should be doing more to enforce basic rights like the minimum wage.”

On his Facebook post, Ali invited employees or clients of the company to come forward with complaints. He did not comment on allegations that the firm failed to pay employees for travel time between appointments or the claim that the company is in breach of minimum wage law.

Ali said Nationwide is a “fully compliant entity in all areas”. He added that the company has been rated “good” by the Care Quality Commission and respective authorities.

HMRC said it could not comment on individual businesses, but a government spokesman said: “HMRC continues to crack down on employers who ignore the law, ensuring that workers receive the wages they are legally entitled to. Workers must be paid at least the national minimum wage or national living wage when travelling from one work assignment to another.”