Two parliamentary committees have questioned Sports Direct boss Mike Ashley about claims the company is underpaying its couriers.
A whistleblower has accused the retailer of manipulating rates for deliveries by Hermes couriers.
By labelling some heavy parcels as "packets", the committees have been told the company underpays on some deliveries.
Examples of these "packets" include an item weighing over 10kg and a bicycle.
The Work and Pensions and the Business Select Committees wrote in their letter to Mr Ashley: "Sports Direct has been reported to us as, 'the most persistent company for this [practice], who almost always send parcels through as 'packets', saving both them and Hermes on courier pay'.
"Given that a not insignificant number of couriers struggle to earn even the National Living Wage, this latest development is hugely troubling to us."
Rachel Reeves, who chairs the Business, Energy and Industrial Strategy Committee, said: "On the face of it, this appears to be a stitch-up between Hermes and Sports Direct to short-change low paid couriers in order to boost company profits."
A spokesperson for Hermes categorically denied the allegations and said the company was happy to answer any questions raised.
He said: "On the rare occasion of an individual label being incorrect, couriers can immediately call our dedicated courier support team, to notify us and change their payment with no questions asked.
"This applies to all our clients and ensures that both Hermes and our couriers receive the correct payment. We have no issues with Sports Direct who remain a valued customer."
Sports Direct has not issued a statement in response to the allegations.
'A new low'
Two years ago an investigation by the Guardian newspaper revealed that Sports Direct warehouse staff were earning below the legal minimum wage.
Workers were required to go through searches at the end of each shift, for which their time was unpaid, and pay was deducted for clocking on for shifts just one minute late.
One MP said last year that the report suggested Sports Direct's working practices "are closer to that of a Victorian workhouse than that of a modern, reputable High Street retailer".
Frank Field, chairman of the Work and Pensions Committee, said: "In the last parliament we heard egregious examples of companies using an essentially bogus classification of self-employment to shirk their responsibilities to those who work for them.
"However, they were simply exploiting loopholes in the law, which we hope to see rectified in this Parliament. If this complaint stands up, it represents a new low in actually deliberately underpaying workers for the work they do."