Customers of every major energy supplier including British Gas, EDF, EON and Octopus Energy are being urged to take a photo of their energy meter before midnight.

That's because the Ofgem energy price cap is set to drop from Tuesday, July 1, meaning the average cost of gas and electricity for everyone on a standard tariff - which is still most UK households - will go down by 7%, or £129, from midnight.

That means if you used gas and electricity on Monday, June 30, but don't take a meter reading tonight, your energy supplier could mistakenly think you used electricity and gas that you use on Tuesday, when prices are lower, on Monday when prices were higher, and inadvertently overcharge you.

Previously, money expert Martin Lewis even urged customers to take a photo of their energy meter with their phone in order to make sure you have proof of the reading you take to be sure you can't be overcharged.

That's because - unless you have a very accurate working smart meter that's updated instantly - the energy firms use estimates of bills.

And if you use a lot of energy on Tuesday but not very much on Monday, the energy firm might average out your usage across the two days and charge you some of it at prices for Monday's electricity - which would be overcharging you because you should have paid the lower rate.

You can record a meter reading at midnight on Monday, June 30 going into Tuesday July 1 but upload it within a few days to your energy supplier and backdate it to June 30.

It used to be that you had to upload the reading on the spot, but energy firms now give you a few days to upload it because customers used to crash websites uploading them all at once.

Martin Lewis has previously explained on an episode of his BBC Sounds and Spotify podcast why you have a few days to actually upload the reading now.

Martin said: "When I first did this I suggested meter reading day and I crashed virtually every energy site.

"You can backdate, go and get a meter reading. For belt and braces, you can take a picture of the meter.

"Most people pay by monthly direct debit, where your average use is taken over the year and it's smoothed out so you don't have big cashflow issues in the winter."