The energy watchdog said it had launched an investigation into Spark after receiving information about poor customer service at the supplier, which has just over 65,000 customers.
Spark, founded around five years ago, was recently featured in a BBC Watchdog programme in which some customers claimed it was "impossible" to switch once they were tied into an agreement with the company.
The supplier, which is based in Selkirk in the Scottish Borders, works with letting agents and tenants are often automatically signed up to Spark through clauses in their tenancy agreements.
Watchdog claimed it had received around 200 complaints from customers of Spark in the past year.
Customers have also reportedly experienced missing bills, struggled to get refunds when in credit and found meter readings not being recorded properly, while suffering difficulties in getting hold of Spark's customer service team.
Ofgem said: "Suppliers are required to receive, handle and process consumer complaints in an efficient and timely manner. Suppliers are also unable to block a customer from switching unless there are valid outstanding charges.
Spark Energy was not immediately available for comment.
It claims on its website the firm aspires to be "the most customer focused energy retailer in the UK". It aims to offer a service that takes the "hassle out of utility bills for tenants whilst offering a great price and taking on the hassle of managing utility services for property managers".