LONDON- Websites for British energy providers crashed on Thursday as customers rushed to submit gas and electricity meter readings a day before prices are set to soar.

From April, the cap on the most widely used tariffs imposed by Britain's energy regulator is due to rise by 54%, meaning energy bills for some 22 million customers will go up by hundreds of pounds.

Suppliers and consumer advice groups have advised people to submit readings for their electric and gas usage before the rise comes into effect on Friday to avoid being charged a higher rate for energy they used before then.

But many angry customers on social media reported problems accessing their accounts with some major providers such as Britain's largest energy supplier British Gas, owned by Centrica , Shell Energy, E.ON, EDF, and Scottish Power, owned by Spanish energy giant Iberdrola .

Firms said their sites were struggling to deal with the surge in visits from customers.

"Right now, we have 2500 calls waiting. On a normal busy day, it'd be 150," Greg Jackson, the chief executive of Octopus Energy, Britain's fifth biggest power supplier, said on Twitter.

"Yesterday, we increased the number of servers we (use) by 250% to help with the expected load."

However, Martin Lewis, founder of consumer website, called for the regulator Ofgem to investigate.

"I'm getting many, many reports of people struggling to submit meter readings," he said on Twitter. "The volume isn't unexpected. Firms really should've been better prepared. It's not good enough."

(Reporting by Michael Holden; editing by William James, Alexandra Hudson)