The energy supplier received only a one star rating from the survey of over 7,000 Which? members between February and March this year for its staff knowledge, phone menu system, politeness, helpfulness and waiting times.
The survey revealed that 21 per cent of Scottish Power electricity customers have spent more than 20 minutes waiting for an answer in the last six months.
Long waiting times were highlighted in the survey along with poor staff knowledge and lengthy phone menus as the reasons why the energy sector ranked joint worst overall with the broadband sector.
Around one in ten energy customers felt call-centre workers had poor or very poor product knowledge, rising to almost one in five, 18 per cent, at big six supplier Npower.
By contrast independent supplier Ovo was the highest rated energy company in the survey receiving five stars across the board.
Which? executive director, Richard Lloyd said: “Unfortunately, poor customer service from call centres has become a 21st century bug-bear for too many people. Customers should vote with their feet if they’re tired of waiting or fed up with the service they get.
“We want the worst offenders in our survey to raise their game by answering the phone quicker and improving staff training to demonstrate they really value their customers’ time.”
The survey revealed that 95 per cent of customers want their calls answered within five minutes by a human voice, and 90 per cent want UK companies to operate UK call centres.
Last week Citizens Advice revealed that Scottish Power had received the most complaints by customers ever in the last quarter of 2014 folowing the introduction of a new billing system.
Scottish Power were not immediately available to comment.