Martin Lewis and his team have urged everyone to check their energy direct debits now.
A third of customers of some of the UK's largest energy suppliers are facing a doubling of their bills.
Martin Lewis' MoneySavingExpert.com say 30 percent of British Gas, Octopus Energy and Shell Energy customers who were in credit and on price-capped tariffs are being "overcharged".
Mr Lewis urged customers "don't accept unfair hikes".
Martin Lewis has issued an urgent plea to those paying their energy bills.
Kirsty O'Connor
The financial expert also gave pointers on how to avoid paying out more than you need to.
"Under firms' licence conditions you have a right to a fair direct debit," Mr Lewis stated.
"Submit an up-to-date meter reading first, then speak to your supplier and politely ask it to justify the rise.
"If it can't, request that it is lowered.
"If it refuses, make a formal complaint and take it to the Energy Ombudsman."
Brits are facing mounting energy costs this year.
Peter Byrne
Downing Street has refused to rule out the possibility of energy bills rising by near £1,000.
Andrew Matthews
Mr Lewis' advice is issued to people who are in credit, and have been on a standard variable tariff for six months or more, but had still seen their direct debit go up too much.
People who were coming off fixed contracts say they had seen their direct debits rise more than anyone else.
The recent rises may have been avoided by those who remained on fixed contracts, but now moving onto the more expensive price cap deal is advised.
Bills are only expected to increase further as the year progresses.
Mr Lewis' tips come as Downing Street refuses to rule out the possibility that energy bills could rise by nearly £1,000 for most customers when the regulator reviews the current cap in the autumn.
It comes after the boss of major energy company ScottishPower called on the Government to take urgent action and help the poorest households months before costs are expected to mount again ahead of the winter months.
Keith Anderson said energy bills are likely to go up by nearly another £1,000 for millions of households around the country when the price cap is reviewed in October.
Asked if the Government recognised this figure, and if it was in the “ballpark” of its expectations, the Prime Minister’s official spokesman did not rule it out.
Mr Anderson said: “I can’t comment on speculation – obviously that … wouldn’t be right for me to do so.
“With Ofgem, I think it’s fair to say that we do expect there to be further increases later in the year, the Chancellor has talked about that and has made clear that we will look to do what we can to help with energy bills in the autumn when we know more about what the prices will (be).”