An investigation into energy supplier Extra Energy has been launched by regulator Ofgem after a record number of complaints were lodged with Citizens Advice.

Ofgem will look into whether Extra Energy broke rules in relation to billing, customer service and complaints handling.

The investigation, to examine if Extra Energy breached Ofgem's standards of conduct, follows the regulator's monitoring of the energy market and it receiving information from Citizens Advice and the Ombudsman regarding the firm.

A flurry of complaints have been made against the company, which has about 400,000 customers, with Citizens Advice revealing last month that Extra Energy had come bottom of its energy supplier league table on handling customer complaints for January to March 2016.

In this period there were 1,682 complaints per 100,000 Extra Energy customers – the highest complaints ratio ever recorded over the course of the league table's five-year history.

Meanwhile, in March, Citizens Advice raised concerns with Ofgem relating to a high level of complaints against Extra Energy, particularly around billing.

And the Ombudsman raised concerns with Ofgem in April, relating to Extra Energy's ability to resolve problems within required timescales.

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Previous problems with Extra Energy spotted by MoneySavers

In May last year, revealed how a number of Extra Energy customers had waited up to nine months to receive their bill while others told us their bills hadn't arrived at all.

Further complaints included final bills not being received by those who switched from Extra Energy, difficulty claiming back credit after switching, and a lack of help from Extra Energy's customer services team.

In a bid to resolve the billing issues, Extra Energy said last year it would change its automated billing system so customers received bills quarterly, rather than every six months.

However, it said it couldn't put a timeframe on when this would take effect as it "takes time to re-programme the automated billing system".

On the FAQ page of its website, Extra Energy currently states that customers paying by direct debit will receive a statement every six months. But if customers choose to pay on receipt of their bill, they will receive this bill by email every quarter.

What happens next?

It's not yet known when the investigation will be finished, but if Ofgem decides Extra Energy did break the rules, it has the power to impose a fine of up to 10% of the company's turnover.

What should I do if I've got a complaint to make?

If you've a gripe with Extra Energy, you should complain to the company. If you're unhappy with the response received or don't get one at all within eight weeks, you can take your complaint to the Ombudsman.

What does Extra Energy say?

Ben Jones, managing director of operations at Extra Energy, says: "We're working closely with Ofgem to resolve these issues. Many have now been fixed and are a thing of the past, thanks to a programme of improvement work we have had in place since early this year.

"However, some challenges do remain and we have presented a clear plan to Ofgem detailing how we will fix them within the next three months. We're confident that we will come out of this stronger than ever, and that our customers will benefit.

"We hope to conclude this process with Ofgem swiftly, and carry on saving households across the UK hundreds of pounds every year."