The head of Big Six energy supplier npower is stepping down from his post.

RWE confirmed Chief Executive Paul Massara will be replaced by Chief Operating Officer (COO) Paul Coffey.

The company made the announcement today following npower’s “unexpectedly” negative mid-year result for 2015.

It saw profits fall 65% and lost more than 300,000 customers in the past year as it struggled with billing problems.

The news also comes as the supplier has been at the bottom of customer satisfaction rankings in recent years.

Dirk Simons, currently RWE Innogy’s Chief Financial Officer (CFO) will become COO.

npower’s CFO Jens Madrian is also stepping down, with his post being taken over by Martin Miklas, currently RWE Polska’s CFO.

RWE said both Mr Massara and Mr Madrian are leaving the company “on their own wishes and by mutual agreement”.

Peter Terium, CEO of RWE AG: “At this time we need a CEO at RWE npower who will focus on fixing the basic process improvements and has a track record of implementing operational process changes. In his previous functions, Paul Coffey has repeatedly demonstrated this expertise.”

Last month regulator Ofgem ordered npower to deliver free energy to customers until it had implemented the rulings of the Energy Ombudsman relating to complaints made about the firm.