Ofgem's investigation into the Big Six energy companies comes after customer complaints relating to energy firms reached the highest number in a single quarter since records began.
The energy suppliers received a total of 1.7 million complaints, with Npower receiving the highest number of 83 complaints for every 1,000 customers.
SSE, British Gas and E.On all received around 30 complaints, with SSE's figures doubling from 13 to 27 on the same time last year, while Scottish Power received the fewest at 13.
"Yet again millions of customers are being let down by poor service from the Big Six energy companies. This has to change," Which? executive director Richard Lloyd said.
More top news
It's not the size of the dog in the fight, it's the size of the fight in the dog, as they say...
Around 7,000 schools have closed to protect children from noxious smog coming from illegal forest fires in neighbouring Indonesia.
Schuler Benson found himself logged into the Facebook account of a woman he'd never met - and six years later she became his wife.