AN INVESTIGATION has been launched into “prolonged customer service failings” at Worcester-based energy giant Npower.

Energy regulator Ofgem has warned the company it must cut the amount of customers affected by late billing from about 400,000 to 100,000 over the next three months or stop all telesales.

The organisation has also launched an investigation into how the power supplier has dealt with customer complaints and said the company could face a financial penalty if it is found to have broken rules around treating customers fairly.

This week it was revealed Npower had received 83 complaints for every 1,000 customers in the first three months of this year, up from 49 per 1,000 in the same period in 2013.

The company has topped polls of complaints figures among the ‘Big Six’ energy suppliers – Npower, British Gas, EDF, Eon, Scottish Power and SSE – since 2012.

Many of these have related to issues in which customers who switched away from Npower, only to receive bills months – in some cases more than a year – later.

In September last year the company admitted a glitch caused by a transfer to a new computer system had affected about 700,000 accounts.

Ofgem’s senior partner in charge of enforcement Sarah Harrison said the organisation had become “increasingly concerned” about the slow progress Npower was making in dealing with its problems.

"Npower's recovery plan has not delivered as far and fast as is necessary,” she said.

“Our analysis of complaints data also raises some serious concerns which will be thoroughly examined in our investigation."

Npower chief executive Paul Massara said the company would cooperate fully with the investigation.

"We are committed to getting things right for our customers but recognise that, despite the progress we have made, our current billing standards have fallen short of where everyone wants them to be,” he said.

Mr Massara added measures were already in place to deal with problems and he was confident it would meet the August target.

"If we have not met this late bill target by then, we will suspend all outbound telesales activity to new customers and not sell additional fuels to existing single fuel customers until we do,” he said.

"I want to reiterate again that our customers should not lose out financially as a direct result of our billing system problems and that, if customers are worried about a high bill, we'll work with them to reach a suitable payment plan."

Energy secretary Ed Davey welcomed the investigation, branding the situation “unacceptable”.