15 million households will see their energy bills increase by more than £100 a year from 1 April despite the introduction of the energy price cap.
Low income families will be particularly worse off according to price comparison service uSwitch.
Collectively the public will be paying £1.67 billion more for its energy each year.
36 different suppliers have raised prices by over 10% on average.
The price cap was announced by the government and implemented by the regulator Ofgem in order to stem rising energy costs.
But many providers have instead opted to raise their standard variable tariffs to meet the cap.
Critics argue it has given firms a target to reach rather than preventing oversized increases or encouraging competition.
Families on lower incomes will be disproportionately affected by the price rises as the increases make up larger proportions of their household income.
Rik Smith, energy expert at uSwitch, says: “These price rises will add more pressure to household budgets across the country, but unfortunately the people that will feel it most are the families who are already struggling to make ends meet.
“This may seem like a cruel April fool’s joke, but it’s no laughing matter. The stark reality is that the very cap that was supposed to protect customers on poor value standard tariffs is now responsible for some of the harshest price increases in recent memory.
“Consumers have just days left before their energy bills will skyrocket. Yet this is at a time when it’s getting cheaper for energy suppliers to buy gas and electricity on the open market, and as a result there are now lots more cheap deals below £1,000 a year available.
“It is vital that households are not lulled into a false sense of security by the idea of a price cap. You can switch to a much cheaper rate in minutes and save over £300 compared to staying on a standard plan that’s priced right up to the level of the cap. So many other bills and taxes are going to cost more from April, but your gas and electricity needn’t be one of them.”
Ofgem has responded to uSwitch’s criticism of the price cap. Its spokesperson says: “The price cap ensures that consumers remain protected from being overcharged for their energy and that any increases in the level are only due to actual rises in energy costs, rather than excess charges from supplier profiteering.
“The cap has significantly reduced consumers’ bills. Our analysis suggests that on average consumers are paying around £75-100 per year less than they would be if the cap was not in place. Households who use more energy, such as large families, are saving even more.
“Consumers who want to save more money can shop around for a better deal.”