Energy bills might be rising but the standard of customer service at some suppliers is going in the other direction, according to Citizens Advice.

The charity's energy watchdog found that customer service is at its worst since 2017, with the highest score for customer service at just 3.85 out of five compared to 3.95 in the previous quarter - June to September 2021 - and 4.40 between October and December 2020.

The average call waiting time was more than five minutes, compared to around four minutes during the same period the previous year.

Customers with the worst-performing suppliers face waiting times of around 15 minutes.

The top suppliers included M&S Energy, EDF Energy, Octopus Energy, Affect Energy and So Energy, while the worst-performing suppliers were Utilita, Boost Power, Ecotricity, Good Energy and Ovo.

The lowest score among these was 1.70 compared to 2.10 in the previous quarter - a 19% fall.

It comes just days after the energy price cap, which will apply until 31 September, increased by 54% to £1,971 a year, a record increase largely driven by wholesale gas prices.

Citizens Advice chief executive Dame Clare Moriarty said: "With millions of people struggling to make ends meet as the cost of living soars, it's more important than ever that people can turn to their energy company for support.

"The drop in customer service standards comes at the worst time possible.

"It's Ofgem's job to make sure companies are delivering a good service. But the current system isn't working.

"A consumer duty focused on making sure customers get good outcomes is what's needed. With bills expected to soar again come October, there's no time to waste."

A Good Energy spokesman said: "We are disappointed with our drop in the Citizens Advice customer service rankings. The last quarter of 2021 was a challenging one for our team and our resources as the energy crisis took hold and we saw many suppliers go bust.

"We are committed to excellent service and it's more important now than ever."

Utilita said: "As a company we really value the fact that the Citizens Advice energy star rating is performed every three months because we operate in a sector where change is constant.

"However, it is also worth acknowledging that the star rating does not fully represent what customers need from an energy supplier today. It does not take into account the financial support we provided on almost 200,000 occasions during the reporting period, nor the millions of households that our award-winning Energy High 5 campaign has helped to cut their energy usage."

Ecotricity founder Dale Vince said: "We've seen a significant increase in customer contact during the energy crisis, with a tendency towards longer calls. Couple that with absences from Covid and the recruitment issues many businesses are facing, (and) it was a challenging time.

"We're working hard to improve performance, and thankfully we've already taken great steps forward since these numbers were recorded last year."