Current legislation means that energy suppliers have up to 65 days after a price change to notify their customers and consumers then have ten days to switch to a cheaper deal.
Commenting on the situation, Dr Fiona Cochrane, senior policy adviser at Which? said: "Surely it's not unreasonable for people to be told in advance if prices are going to go up?"
"It's time for the government to step in and make sure Ofgem delivers on its mission to put consumers first," she added.
Last week, Ofgem proposed increasing the period in which consumers could switch energy supplier from ten to 20 days.
However, Which? said this would still favour suppliers.
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