Energy supplier Solarplicity has been banned temporarily from taking on new customers due to its poor switching process and customer service.
Energy regulator Ofgem has also banned the company from increasing vulnerable customers' direct debits.
Ofgem warned that the firm, which has 60,000 customers, could have its licence revoked if it did not improve.
Solarplicity said Ofgem's decision "disregards the vast improvements that we have made to our customer service".
Ofgem said it had been seeking improvements to Solarplicity's customer service, "which has been poor for a number of months".
It said that between March 2018 and September 2018, "there was an unacceptably high proportion of calls abandoned and unacceptably long call waiting times".
"Whilst call handling has improved, Ofgem has not seen the required improvements elsewhere."
The regulator said that the ban would remain in place for three months unless it saw significant improvements in the company's customer service and switching process.
Mary Starks, executive director of consumers and markets at Ofgem, said: "Solarplicity must get its house in order and provide a level of service that its customers expect.
"If not, Ofgem will take the necessary steps to ensure customers are further protected and will take the relevant action needed to do this, which may result in its licence being revoked."
In response to Ofgem's decision, a spokesperson for Solarplicity said: "We are committed to meeting the expectations of every single Solarplicity customer, but Ofgem's decision, which was made on old historical data, disregards the vast improvements that we have made to our customer service.
The spokesperson said investment had "significantly strengthened our account switching service, and the vast majority of our customers are able to switch their accounts well within the 21 day-limit."
Gillian Guy, chief executive of Citizens Advice, said: "Solarplicity languishes at the bottom of our energy star rating league table, which assesses suppliers' standards of customer care.
"We've shared our concerns about the service consumers are getting with both Solarplicity and Ofgem. It's now up to Solarplicity to address these matters and start delivering for its customers.
"This provisional order highlights the continuing problem of struggling companies operating in the energy market. Ofgem must continue to use the powers at its disposal and take action against failing firms."
Despite efforts to encourage the emergence of new energy firms to challenge the dominance of the "big six" companies, many of the smaller suppliers have struggled.