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, ended days of speculation by confirming that its 11,500 strong workforce will be reduced by almost a fifth.

The two main RWE businesses in the UK made an aggregate loss of £154 million in 2015 - £99 million at npower and £55 million at RWE Generation.

The job cuts will affect a mixture of direct and indirectly employed staff.

A two-year recovery plan was announced to deliver a "robust business" built on lower costs, simplicity, high-quality customer service, and ready for the challenges of the future.

A statement said: "Npower 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."

Unison general secretary Dave Prentis said: "These huge job losses will come as a devastating blow to the workforce.

"Npower has been in trouble for some time thanks to poor decision-making at the very top, and workers are now paying the price. The company's failure to invest properly in new systems has left it with one of the worst customer service records in the business."

Paul Coffey, chief executive of RWE npower, said: "Npower results continue the trend seen earlier in 2015, but they are nonetheless extremely disappointing and we are starting a two-year process to fix them.

"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.

"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.

"We shared the outcome of this review yesterday with employees. By 2018, around 2,400 fewer people will support npower overall through a mix of those who work directly and indirectly for npower."

The job cuts will be spread across offices, including some in the Midlands, and are not "site specific".

But npower confirmed the closure of an office in Burton, which employs around 200 staff.

There will be no job cuts at any of npower's power stations.

Unison general secretary Dave Prentis added: "At the weekend npower employees had to find out from the media that their jobs were on the line. Today's poor results mean yet more uncertainty for the workforce.

"Npower bosses are compounding the anxiety for staff by refusing to meet the unions nationally to discuss this so-called recovery plan and to talk about how to protect jobs and avoid compulsory redundancies. We're calling for an emergency meeting so we can work jointly on finding a way out of the mess the company currently finds itself in.

"The workforce will be hoping that the worst is now behind npower, and that in future employees will be the first to know if jobs are under threat."

Unite, which represents meter readers, revenue protection officers, gas fitters and electricians, warned npower against "self-defeating" cuts.

The job losses include 450 recently announced by npower in the firm's home and social team with the first tranche of redundancies expected at the end of this year, followed by more at the end of 2017, said the union.

Talks on the detail of today's announcement are expected to take place between unions and npower over the coming weeks.

Unite national officer Kevin Coyne said: "Confirmation of these job losses on this scale is a bitter blow for workers, their communities and the wider energy sector.

"Npower's ability to repair its tarnished image and keep the lights on for its customers will hinge on a workforce which has worked hard over the past year to turn the company around.

"They should not be made to pay for the company's past mismanagement with their livelihoods and we would urge npower not to cut too far, too fast and lose vital skills which could undermine it in the long term."

Energy and Climate Change Secretary Amber Rudd said: "All job losses are regrettable and I have every sympathy for the people who are affected by them. Support will be available from Jobcentre Plus to help those facing redundancy move into new jobs as quickly as possible.

"It's really important that consumers are able to trust the energy companies and know they're getting a good deal. That's why we're promoting more competition and switching so that energy companies realise they have to put their customers first."

Eamon O'Hearn, GMB national officer, said: "It is disappointing that hard-working members, many of whom have experienced significant disruption and uncertainty over the past 12 months, are set for yet more uncertainty over the next 12.

"The company has failed to clarify where any job losses are likely to occur, meaning that members will go home tonight none the wiser.

"Our members, in particular those in customer-facing roles, have been instrumental in helping the company turn around the retail business and we believe they deserve to know the full picture as soon as possible.

"RWE's German workforce would not be treated in this manner."