COMPLAINTS about big energy firms Npower and Scottish Power doubled in the first quarter of 2014 after new billing systems caused problems for customers.

Citizens Advice Bureau (CAB) said yesterday that complaints about Npower rose from 307 per 100,000 customers in the last quarter of 2013 to 592 between January and March.

Complaints about Scottish Power increased from 100 per 100,000 customers to 198 in the first quarter.

Last month watchdog Ofgem warned Npower to resolve its billing problems by the end of August or halt all telephone sales to new customers.

CAB chief executive Gillian Guy said: “The knock-on effect of poor billing systems can turn household budgets upside down.

“Many people do not have the spare cash to cover the cost of a large bill that suddenly lands on their doorstep.”

Npower’s domestic business director Roger Hattam said: “We have been working hard to fix our billing system issues.”