Plans by energy giant npower to cut 2,400 jobs in the UK amid heavy financial losses have been described as a “devastating blow” to its workers.

The company, owned by German group RWE, yesterday ended days of speculation by announcing that its 11,500-strong workforce will be reduced by almost a fifth.

Confirmation came alongside annual results showing that the two main RWE businesses in the UK made an aggregate loss of £154million in 2015 – £99m at npower and £55m at RWE Generation.

The job cuts will affect a mix of direct and indirectly employed staff and form part of a two-year recovery plan which RWE said would deliver a “robust business” built on lower costs, simplicity and high-quality customer service, and ready for the challenges of the future.

In a statement, npower said it “takes its responsibilities to its employees seriously and will consult fully with affected employees and with unions over its proposals for the future of the business”.

However, Dave Prentis, general secretary of the Unison union, said: “These huge job losses will come as a devastating blow to the workforce.”

He added that workers were paying the price for “poor decision-making at the very top”, with the company’s failure to invest properly in new systems having left it with “one of the worst customer service records in the business”.

Paul Coffey, chief executive of RWE npower, said the results continued a trend seen earlier in 2015, but were nonetheless “extremely disappointing”.

“They show a business that tried to do too much, too soon, while not focusing enough on the fundamentals in a constantly changing market,” he said.

“This led to over-complicated processes and procedures resulting in unhappy customers, too many complaints and extra costs to put things right.

“These issues are not insurmountable. Over the past few months, we have looked at every part of npower, and over the next two years we’re fundamentally changing how the company operates,” said Mr Coffey.