Ofgem has extended its ban on Iresa, saying the company has so far failed to address the regulator's concerns.
Ofgem handed Iresa a temporary ban in March, preventing the company from signing up new customers, accepting one-off payments and increasing its direct debits.
The regulator said it would take further action if Iresa did not extend its call centre hours, clear its email backlog, and respond to customer emails within five working days.
Ofgem also demanded the company make a log of its customers' problems and put its vulnerable customers on a priority register.
In a statement on Wednesday, Ofgem said Iresa had "significantly reduced" its call waiting times, but had not made sufficient progress on its email backlog.
Iresa has also improved its handling of complaints, and its management of vulnerable customers, but needed to make further progress and prove that it could sustain this progress for a reasonable length of time.
Iresa's licence could be revoked if it does not meet Ofgem's demands.
Dermot Nolan, chief executive of Ofgem, said: "Iresa continues to let its customers down by failing to provide a level of customer service its customers deserve.
"Although it has made some improvements since we issued the provisional order in March, the supplier has been unable to meet all the requirements we set it three months ago to get its house in order."