A spokeswoman for Nisa-Today's confirmed that the group had written to
T-Mobile asking for an explanation of how any changes will impact on retail members. Other retail groups are also believed to be in talks with T-Mobile.
Another retailer angered by T-Mobile's move said the main worry was that it could lead to a flurry of similar demands from the other service providers.
One London retailer contacted by C-Store said that top-ups were a key service for him, not only for the commission but to encourage footfall. He said that any reduction in commission would hit him hard. He added: "We make 4.5% on top-ups so it's pretty clear that we cannot afford for this to be reduced any further."
T-Mobile said the reduction was part of its strategy to put the customer first.