Energywatch says almost half of the complaints it has received about Opus Energy this year have come from publicans.
The energy watchdog says it has had 59 complaints from customers who say they are unhappy about the service they are receiving from the electricity supplier and 25 have come from licensees.
The Publican has itself been inundated with calls from distraught licensees.
Some are anxious they have signed up to an estimated monthly rate only to find their costs have soared and that they are locked into a commercial contract, with no way of changing supplier.
Paul Savage, business services manager at Energywatch, is concerned about the vulnerability of pubs. "A significant number of these complaints are from pubs," he said. "There are some issues with the way energy companies, including Opus, sell energy.
"We are happy to see that Opus has responded positively and the number of complaints is coming down." He warned that pubs should be aware that commercial contracts have no cooling-off period and are fixed for an agreed period.
A spokesman for Opus Energy said: "We are concerned that 59 of our customers have made complaints to Energywatch about our service. We are working closely with Energywatch and the customers involved to resolve these issues.
"As Energywatch has confirmed, we have recently improved our processes following customer feedback and are constantly working to minimise the number of complaints we receive.
"While, obviously, every complaint is one complaint too many, the number of 59 outstanding complaints should be put in the context of the number of customers we supply. Over the past few years we have supplied more than 30,000 customers, the vast majority of whom have made significant cost savings through Opus and are completely satisfied with our service."
What is Energywatch?
Energywatch is the independent gas and electricity watchdog. It was set up in November 2000 through the Utility Act to protect and promote the interests of all gas and electricity consumers.
It provides free, impartial information and advice and takes up complaints on behalf of consumers and businesses directly with their energy companies.
Go to www.energywatch.co.uk or call the helpine on 08459 060708