COMPLAINTS to Worcester-based energy giant Npower and the five other big six power suppliers soared to the highest-ever level during the first half of this year.

In the last six months of 2013 10,598 complaints were made to the companies, but this more than doubled to an all-time high of 22,671 between January and the end of June this year.

Npower has consistently received more complaints than the other five UK energy ‘giants’ – British Gas, EDF, Eon, Scottish Power and SSE – since 2012, with the latest figures showing it received 83 complaints per 1,000 customers in the first three months of this year.

The majority of these are due to bills arriving late and energy regulator Ofgem has warned the company it must cut the amount of customers affected by the issue from 400,000 to 100,000 by the end of August 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.

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.

An Npower spokesman said the company was focusing on removing the backlog of complaints and resolving billing issues and was on target to meet the August deadline.

A spokesman from the Department of Energy and Climate Change branded the figures “unacceptable”.

"Energy companies need to realise that people will not tolerate poor service and are switching suppliers in unprecedented numbers, particularly to small suppliers whose numbers have nearly trebled since 2010,” he said.

Chief energy ombudsman Lewis Shand Smith put the spike in complaints down to both an increase in the cost of living as well as customers becoming more aware of their rights.

"Addressing these concerns is crucial to restoring consumer confidence in the sector," he said.

A spokesman from Energy UK – the trade association for the UK’s power industry – said: "The energy industry works hard to provide the best service for its customers, but in an industry serving 27 million households sometimes things go wrong.

"However, no one wants to see complaints rise and each complaint is taken very seriously with companies working hard and investing in resources and new systems to resolve issues as quickly as possible.

“Most complaints are dealt with by the end of the next working day with no more than a phone call.

"The industry will be consulting stakeholders this summer about the complaints it cannot fix in 48 hours.

"If any customer is concerned or has a problem with their energy supplier they should speak to the companies first.”