Customers trying to get through to their energy company are being kept on hold for up to 30 minutes.
And the performance of some of the biggest suppliers is getting worse, despite the fact bills for heat and light are rising.
Researchers from Which? made 312 calls to 26 energy companies and found huge differences in the pick-up time.
The biggest company, British Gas, was the second slowest with a time on hold of 9mins 33secs. This is more than double the wait of 4mins 14secs seen a year ago
Delays on customer service lines were particularly bad at most of the ‘Big Six’ suppliers – British Gas, SSE, Npower, E.on, EDF and Scottish Power – which dominate the market.
The longest average wait to get to speak to a real person was with the German-owned E.on at 14 minutes and 18 seconds.
The biggest company, British Gas, was the second slowest with a time on hold of 9mins 33secs. This is more than double the wait of 4mins 14secs seen a year ago.
Third slowest was SSE with an average of 8mins 18secs. The company is in merger talks with Npower, which has a poor record on customer service and kept callers waiting for 7mins 36secs.
The longest average wait to get to speak to a real person was with the German-owned E.on at 14 minutes and 18 seconds
Scottish Power was the fastest of the Big Six, keeping researchers waiting under 2mins.
The fastest was the green energy supplier Bulb at just 27secs.
The worst performer of the 26 firms, was small supplier Spark Energy which kept a researcher on hold for more than 30mins.
Alex Neill of Which?, said: ‘It’s unacceptable that the worst performing energy suppliers keep their customers waiting on the phone for so long.
‘If you are unhappy with the service you are getting from your energy provider you should consider switching to another.’
An E.on spokesman said: ‘Clearly, these aren’t the findings we want to see from the Which? snapshot. We’ve since taken steps to deliver improvements and believe these have already had a positive impact since this survey was taken a couple of months ago.’