'Big six' energy firm npower has confirmed 2,400 jobs are to go in the UK after it racked up an operating loss of £106m in 2015 - a year in which it lost 7% of its domestic customers.

Its German parent, RWE (LSE: 0FUZ.L - news) , said a "radical restructuring" of the UK supply business was getting underway, just days after Sky News reported the shake-up would result in 20% of npower's UK workforce losing their jobs.

It (Other OTC: ITGL - news) was a response to customer numbers diving from 5.13 million to 4.77 million - with npower blaming billing problems and competition.

RWE said of the operating loss at npower: "The main reason for this is serious process and system-related problems in customer billing. Substantial earnings shortfalls also stemmed from the fact that residential and commercial customers switched providers or we were only able to retain such customers by offering them contracts with more favourable conditions.

In December, the company was ordered to pay a £26m settlement by the energy regulator, Ofgem, for "failing to treat customers fairly" - the second such fine it has had imposed on it.

Paul Coffey, npower's chief executive, said the results were "extremely disappointing" and it would take "a two-year process" to fix.

"They show a business that tried to do too much, too soon while not focusing enough on the fundamentals in a constantly changing market. This led to over complicated processes and procedures resulting in unhappy customers, too many complaints and extra costs to put things right."

He added: "Energy should be simple for our customers and we have complicated it. Our plan is to create an npower that delivers better service, is more attractive to customers and better prepared for future opportunities – all at lower costs".

It released its results days before UK competition regulators are tipped to announce a series of measures aimed at making the energy market more transparent amid criticism that customers have been paying over the odds for their energy.

Npower, along with 'big six' rivals such as British Gas and SSE (LSE: SSE.L - news) have announced a round of price cuts ‎in recent months but have been criticised by ministers and consumer groups for failing to go further on trimming standard tariffs, given sharp falls in wholesale costs over the past year.