Couriers at one of the UK's largest delivery firms feel they are "treated like dirt" and have faced losing work for attending hospital appointments or tending to sick relatives, the Prime Minister has been told.
Details of complaints made about Hermes by current and former staff were revealed in a letter sent to Theresa May by the chairman of the Commons Work and Pensions Select Committee, Frank Field.
Mr Field urged the Prime Minister to launch an investigation into working practices at Hermes UK, alleging that couriers were being paid below the minimum wage.
Hermes said it would look into the allegations made by Mr Field but insisted it had an "extremely committed and loyal network of couriers who enjoy their work and deliver high standards of service in return for for good rates of pay".
Senior Labour MP Mr Field said he had been contacted in recent days by a number of current and former workers - all of whom were contracted by Hermes UK on a self-employed basis - about aspects of the firm's operating procedures "which, in the words of one courier, make them feel as though they are 'being treated like dirt'".
In one case, two couriers whose six-year-old son died were "hounded" to return to work, Mr Field said.
The boy was taken to hospital for an emergency leg amputation and placed on life support but his parents were told that "if they did not return to work immediately, they would have their rounds taken off them".
The child subsequently died "yet still they were being hounded to return to work, or face losing their delivery rounds", Mr Field said.
The MP also claimed that, once fuel costs are factored in, couriers are paid less than the legal minimum wage for their work.
One courier who lost work because their vehicle had broken down was told by a manager "you should have a spare car on the drive then, shouldn't you", Mr Field claimed.
He also claimed couriers reported they could only book time off work if they had arranged for, and trained, another worker to provide cover.
"In one case, a courier with three children gave two months' notice of her wish to take off a bank holiday Monday over the Christmas period so that they could visit a close family friend who had been diagnosed with cancer," Mr Field said. "Their manager responded by taking away their delivery round."
Mr Field said he had been told of couriers returning to work in the middle of a holiday "for fear of the consequences of not doing so".
He also raised concerns about seven-day working and whether Hermes UK was meeting its tax obligations.
The MP told the Prime Minister: "From what the couriers have told me, it seems as though they enjoy none of the benefits that are supposed to come from flexible self-employment. But they do shoulder almost all of the risks and insecurity.
"They appear to have no real choice at all over their own working patterns, and no real control over their own lives.
"Each of these main areas of concern will, I hope, prompt the Government to inquire into whether Hermes is complying with its legal obligations. But I hope also that the Government will take a serious look at whether the practices that have been reported here are undermining your agenda for protecting the interests of low-paid workers in our country."
A Hermes spokeswoman said: "We are aware that Mr Field has written to the Prime Minister with a number of allegations regarding our UK operations. We will investigate the specific cases raised in the letter to establish the facts of each case.
"We have operated in the UK for many years and have grown to a network of over 10,000 couriers. Our growth and that of the sector has been driven by the change in purchasing habits in the UK.
"We are proud to have an extremely committed and loyal network of couriers who enjoy their work and deliver high standards of service in return for for good rates of pay.
"We will work with the Government and delivery industry to look at how we can best serve the needs of couriers, customers and consumers."