Energy customers around the country have been struggling to submit gas and electricity meter readings, a day before prices are set to soar when the price cap is raised.
Millions of households will see their energy bills rise to a typical £1,971 a year when the cap is changed on 1 April.
The 54% increase announced by regulator Ofgem, which will add £693 to the annual costs of a typical household, follows a 12% rise in October.
Users reported being unable to access the websites of energy giants such as British Gas, SSE, E.ON, and EDF, to submit their meter readings.
Consumer advice groups have urged Britons to take a reading and submit it to their provider before the price cap is raised on Friday, to avoid being charged a higher rate for energy they used before then.
Some users complained that they were unable to log in to their accounts, while others said their provider’s website was offline.
Scottish Power, owned by Spanish energy giant Iberdrola, said it was working to resolve an issue with its website and told customers they could submit meter readings via an automated telephone line instead, Reuters reported.
Sky has contacted British Gas, SSE, E.ON and EDF for comment.