That is the warning from comparison website MoneySuperMarket.com, which said that 22 energy tariffs from the biggest suppliers will expire at the end of April.
Clare Francis from the site said: "Customers on these products face the prospect of an automatic price hike of up to £210 on some tariffs if they don't act soon to switch to a better priced deal."
When a fixed-rate deal comes to an end, providers typically roll customers on to an alternative variable product - and, as with the banks, more often than not this is your supplier's standard and least attractive rate.
For instance, EDF Energy's popular Blue+ Price Promise April 2014 tariff reverts to a standard tariff costing on average £210 more per year. But there is no exit fee, so there is no need to wait until the fixed term ends - you can apply for a new product now.
However, many tariffs do trigger a penalty for switching out before the fixed term ends, so moves should be timed carefully to avoid exit fees which can go up to £70.
The typical switch time is as long as six weeks, so if your deal expires at the end of next month, start looking around now.
Francis said: "It might not even mean switching provider, as you may find your existing provider is offering the cheapest tariff."
Mark Todd, director of comparison site energyhelpline, said small suppliers may have taken 150,000 customers from the "Big Six" last month, taking the total to 600,000 since the big round of price rises kicked in this winter.
He said small suppliers such as Ovo, first:utility and Green Star Energy can offer cheaper tariffs "as wholesale prices have not really gone anywhere in the last three years" yet gas and electricity charges are up around 30%.
Todd said switching savings vary from £184 to £286 a year, for a typical home.
However, price may not be the only consideration when deciding whether or not to stick with your provider or jump ship.
Last year, E.ON received 10 complaints per thousand customers, SSE received 15, British Gas 19 and ScottishPower 20. But way at the bottom of the class were EDF and npower, both with almost 30.
In the complaints league, the best performers were Co-operative Energy and Good Energy, with fewer than four complaints per thousand customers, and Ecotricity with fewer than one.