This could result in a huge and possibly inaccurate catch-up bill down the line once the customer's meter is read, because the supplier will not know which units of energy were used before the price rise.
"Consumers could be paying higher prices for gas or electricity used in the past, while for companies it results in a very nice windfall," Robert Hammond, of Consumer Focus told the BBC.
A colleague at the group explained that the situation would only really change when so-called "smart" meters were rolled out across the country.
These devices offer a two-way communication between the energy supplier and household, offering real-time information on gas and electricity use and giving more accurate bills.
They are currently being trialled by the energy regulator Ofgem.
In the meantime, Consumer Focus urges customers to check their bills for the letter "E" which means that the charge is estimated.
And if that is the case to take a reading of their meter and contact their supplier.
The Energy Retail Association said the industry was not profiteering from the practice but did warn customers not to rely on estimated bills.
"It's important to get a correct bill rather than an estimated bill," said Gary Felgate at the Energy Retail Association.
The best way of doing this is to take a reading of the meter on the first day of a price rise taking effect, Consumer Focus added.
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