New billing systems meant that customer complaints almost doubled for Scottish Power and npower in the first quarter of 2014.
The number of complaints during the first quarter of 2014 from customers of npower and Scottish Power doubled thanks to problematic new billing systems, according to Citizens Advice.
Complaints about npower rose from 306.8 for every 100,000 customers during the closing months of 2013, to 592.4 between January and March - or one complaint for every 168.8 customers.
Scottish Power received 197.7 complaints per every 100,000 customers – up from 100.5, stated Citizens Advice and Citizens Advice Scotland.
Gillian Guy, chief executive of Citizens Advice, 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.
"While we recognise npower is receiving more complaints because it is starting to get over some of the earlier issues, it needs to do more to stop consumers' problems escalating. Offering repayment plans and discussing ways they can help consumers from the off will nip issues in the bud and remove the need to complain."
Scottish Power has also had similar problems with a new billing system, but this is relatively new development and needs nipping in the bud, noted Guy.
"Scottish Power has an opportunity to learn from other suppliers' new billing system failures and address these problems now so more consumers won't have cause for complaint," she said.
Complaints about all the 'big six' energy firms rose in the first quarter - Scottish Power and npower saw the largest increases - the overall effect of which Guy described as a "concern".
"A rise in complaints about all suppliers is concerning. Suppliers won't win the trust of customers back unless they show they understand what consumers need, recognise the financial pressures many people are under and are able to sort out problems quickly. This is something that all suppliers can act on now," said Guy.
Supplier – Complaints per 100,000 customers from January to March 2014
1. SSE – 34.1
2. British Gas – 75.4
3. E.ON – 83.6
4. EDF – 84.5
5. Scottish Power – 197.7
6. Npower – 592.4
Average - 151