Labour has attacked the Prime Minister for failing to help families struggling with rising energy bills by freezing price rises.
It follows the announcement this week by Southern Electric (SSE) that it is raising its prices by 8.2%.
Other firms in the so-called 'Big Six' are likely to follow suit.
Shadow Energy Secretary Caroline Flint told Sky's Murnaghan programme that David Cameron has two years before the next election to make a decision on whether to help householders and businesses.
The PM has been accused of protecting a market which introduces price rises annually.
The Labour party has promised to freeze prices until 2017 if the party wins 2015's general election.
Ms Flint said: "I would urge David Cameron at look at this now and question whether or not these price rises are justified."
She added that she would also like the main companies to be pressurised into breaking away from "the herd" and lowering prices.
Labour leader Ed Miliband wrote in the Sunday Mirror that Mr Cameron "is the man who lets energy bills go up by £300 for families instead of reforming the energy market".
"He is always weak when it comes to standing up to the strong. And this is the week when the country could see it."
The Energy Secretary Ed Davey told the BBC's Andrew Marr Show consumers are likely to see higher bills as international wholesale gas prices rise.
"The gas price has gone up by 50% over the last five years and over 50% of the bill is wholesale gas price. We have also seen a big increase in the network costs because we have got to replace ageing network. These are costs which are impossible to avoid."
However he ruled out cuts to so-called "green" taxes which critics blame for pushing up prices unnecessarily.
"Most of them are actually social policies to help the fuel-poor manage their bills. I don't think we want to get rid of those," he said.
"Moreover we need to help people who aren't necessarily fuel-poor but want to do energy efficiency measures to reduce their bills. It would be silly to get rid of that aspect. If you actually look at the part of the so-called green taxes which is funding renewable energy/low carbon energies it's only 4% of the bill."
But he added that a "massive" wave of investment from China, Japan and Korea would secure the UK's power supply into the future.
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