Customers of Scottish Power were left waiting to speak to a call handler for an average of more than 20 minutes - the worst wait of all energy providers - an investigation has revealed.
Which? made 432 calls to 36 energy suppliers to reveal how long customers were left waiting before speaking to a member of staff.
A fifth of suppliers kept customers on hold for longer than 10 minutes on average - however, Scottish Power customers waited for an average of 21 minutes and 24 seconds before calls were answered - 20 minutes slower than the fastest company, So Energy, which answered calls in just 38 seconds. On some occasions, it took more than 30 minutes for calls to be answered.
More than half of the suppliers Which? investigated had an average call waiting time of more than five minutes, including four of the Big Six.
Natalie Hitchins, Which? head of home products and services, said: “Energy customers should be able to expect good customer service from their supplier, so it is unacceptable that some people are facing waits of half an hour or more just to speak with an adviser.
“If you are tired of poor customer service and wasted time on the phone, then you should switch to a provider that can better meet your needs, and potentially save you hundreds of pounds.”
A ScottishPower spokesman said: “Nobody wants to be at the bottom of this type of list and we’re already working hard to regain our top spot.”