Scottish Power has been rated worst for customer service in a ranking of the UK's biggest brands.
The energy supplier had the lowest rating of 100 brands, with just 59% of the 3,500 consumers questioned in the Which? survey rating it positively.
Customers reported "useless service and unhelpful standardised replies".
The Which? survey found that Scottish Power's competitor npower was not rated much more highly at 61%.
Other energy sector firms also performed badly, with SSE managing the top score of 74%, followed by British Gas and EDF Energy (both 71%).
Telecoms giants BT (63%), TalkTalk (64%), Vodafone (66%) and EE (68%) also had a "worrying trend" of poor performance.
In reply Scottish Power said:
"Last year all our customer accounts were migrated on to a new IT system, which resulted in a very busy period as disappointingly we experienced more problems with the new system than we would have liked.
"The new system is very important as it ensures that Scottish Power customers will be able to take full advantage of all the benefits we anticipate from the roll-out of smart meters in the coming years, and it will allow us to continue adding innovative features to make it easier for customers to manage their accounts.
"To restore our service we have recruited 500 additional customer advisers to our UK call centres and, since the survey was conducted, our average speed of answer has improved by 72% from 193 seconds in 2014 to 54 seconds over the last three months and we have also seen a 50% reduction in our regulated complaints."