Ofgem's finding that aspects of energy firms' behaviour "would appear to be consistent with tacit coordination" has been denied by Centrica.
If the temperatures weren't enough to make you shiver, the extra costs from cranking up the heating may do.
Increasing 'non-commodity costs' have prompted British Gas to warn that household bills will rise too.
The boss of Centrica, which owns British Gas, told ITV News the 'Big Six' energy firms are "absolutely not a cartel" after Ofgem's review found evidence of possible tacit co-ordination.
The energy watchdog said it did not find evidence of explicit collusion between the firms but said the timing and size of price announcements were part of the evidence that showed possible tacit co-ordination.
Sam Laidlaw told ITV News he "refutes totally" the idea that the 'Big Six' are a cartel and claimed that the energy market is "vibrant" and "very competitive."
ITV News Business Editor Joel Hills has tweeted:
The boss of Centrica tells me he's pulling plans for new gas fired power station in England. On hold until CMA investigation ends.
The chief executive of Centrica, which owns British Gas, has welcomed the major competition inquiry into energy firms but said a lengthy review could damage investment when the market's "security is being seriously challenged."
Sam Laidlaw said: "Anything that clears the air and helps rebuild trust in the industry must be a good thing. Britain's energy market is highly competitive and we believe that a full independent review by a respected regulatory authority would demonstrate precisely that.
"Competition is working, providing choice for consumers and some of the lowest prices in Europe.
"We hope that a lengthy review process will not damage confidence in the market, when over £100 billion of investment in new infrastructure is needed.
"A prolonged period of uncertainty could damage investment at a time when Britain's energy security is being seriously challenged."
Three energy firms have said they have not yet made a decision about bonuses for their respective chief executives.
A spokesperson for npower said bonuses would be "reviewed in the new year", SSE said it would "take all factors into consideration including the general external environment" and Scottish Power said it "evaluates individual performance against the agreed criteria at the end of the financial year."
Centrica boss Sam Laidlaw announced he has decided not to take his bonus this year.
Are energy companies about to follow where banks went before?
This lunchtime, the boss of the biggest of the big 6, Centrica, told me that he had decided not to take a bonus this year.
It's important to remember the firm's board won't have made that decision yet anyway, but he is removing himself from even being considered for one.
High pay for bankers certainly met with massive public anger during the worst of the financial crisis.
But it will be interesting to see if Laidlaw's decision makes much difference to anxiety over bills, and whether the bosses of other energy companies follow.
The head of Centrica, which owns British Gas, has decided not to take his bonus this year as anger over soaring household energy bills continue.
Consumers' trust in the energy sector is at an "all time low" and there was an urgent need to rebuild it, chief executive Sam Laidlaw said at the CBI annual conference in London.
He announced his decision not to take bonus and added that there was a need for leadership in the current environment which had to be balanced with being able to attract employees.
"Just to continue in this world where households are under pressure, and assume it is business as normal, is not the way thoughtful remuneration committees think about it," Mr Laidlaw said.
He also repeated the industry's pledge to cut bills if the Government switches environment and social costs to other forms of taxation.
British Gas owner Centrica hopes a scheme to offer free electricity on Saturdays will encourage households to use electrical appliances at the weekend, when industrial demand is lower.
The development came as Centrica hinted at further price rises as British Gas announced profits from its residential arm rose by 3.2 percent, thanks to the freezing spring weather.
Earnings increased to £356 million during the first half of 2013, as the company cashed in on the bitterly cold temperatures after raising tariffs six percent at the end of last year
British Gas has said it could offer free electricity on Saturdays in a bid to reduce demand on weekdays.
The energy firm has announced plans to trial the initiative and could make it available to customers by mid-2014.
British Gas owner Centrica already offers a Free Energy Saturdays tariff to customers in the US.
A spokesman for Centrica said: "We are looking to see whether we can introduce it into the UK for customers with a smart meter.
"Once we have trialled it, if it works then we could potentially offer it as a product for customers from the middle of next year."
British Gas is poised to offer customers free electricity on Saturdays, according to a report in the Financial Times (£).
Centrica, which owns British Gas, hopes the tariffs would encourage customers to concentrate their energy use at weekends when overall demand is low.
Proposals for the new energy tariff, which could be introduced next year, are similar to deals offered by American provider Direct Energy, a sister company of British Gas.