These website outages tend to be due to dramatically increased web visits, Octopus Energy said. A spokesman said the company had added additional capacity to bolster servers.

However, customers still have another two days to submit their readings with Octopus, he said. “It’s worth noting that Octopus Energy’s price change will actually not come into effect until midnight on April 2. If customers are keen to give us a meter reading to coincide with the change, the best day to collect it is on April 2.”

Those who take a photograph of their readings on Saturday will have a week to submit it via their online account, he added. 

E.On that any meter readings customers take today can be updated to their account online in the coming days. In response to the firm’s Martin Lewis tweet, an E.On spokesman said: “This was an ill-considered and off the cuff remark made by one of our energy specialists and in no way reflects our position. We have an excellent working relationship with Martin Lewis and his team, and we apologise for any offence caused.” 

Gareth Kloet of GoCompare, the price comparison website, said: “It’s absolutely worth taking a meter reading before these increases come into effect. By doing this, it simply means your energy company cannot charge you at the higher rate for any units that have been used prior to April 1.”

EDF Energy and SSE have been approached for comment. British Gas said no customers would be left out of pocket as a result of this system issue.

A spokesman at Shell Energy said their teams were “working hard to keep the website online” as they deal with a surge in the number of customers trying to access their account.