Households reeling from the latest rise in energy bills could enjoy their current lower prices for most of the winter, simply by using a little-known regulatory rule.

Thanks to an obligation put on the energy companies by the regulator, Ofgem, customers are able to reject their supplier's price rises while switching to another energy provider – but they have to do so by contacting their supplier within 20 working days of receiving the letter.

Energy customers telling their supplier they intend to switch to another one can enjoy the pre-price increase tariff until the switch goes through. This process typically takes six weeks.

So far this winter British Gas, Scottish & Southern and Scottish Power have announced increases to gas and electricity prices – mostly in the region of 7%-9%. Last Friday Npower became the latest supplier to increase its prices. Npower's gas and electricity prices will rise by 5.1%. The increase, which adds around £60 to the average, annual duel fuel bill, comes into force on 4 January.

However, under Ofgem rules, the energy companies cannot enforce their price rises if a customer tells them that they reject the rise within 20 working days of receiving the notification of the increase.

At present, energy companies can send letters about price increases up to 65 working days after the increase has been applied.

Once customers have informed the supplier that they reject the price rise, they must make arrangements to switch supplier within 15 working days. The switch will then take around six weeks, during which time the customer will continue to enjoy the lower price.

Depending on how long it takes suppliers to inform customers about price changes, this entire process could last as long as 20 weeks, during which time customers could continue to enjoy their current energy prices. Both E.ON and EDF are yet to raise prices. The latter has committed not to increase its tariffs until 1 March at the earliest.

For some customers who have already been told their prices are rising, the news will have come too late. However, millions of letters are still to be sent out, allowing the opt out.

Joe Malinowski, founder of energy price comparison website TheEnergyShop.com, says: "These energy price rises are happening, irrespective of whether they are justified or not. And consumers need to take action now to protect themselves. If you've been notified of a price increase, the first thing to do is to put your supplier on notice that you are going to leave. That way you can prevent the increase from kicking in if you start a switch to a different supplier within two weeks.

"Then take a good look around the market for a deal that really suits you. You could quite easily pick up a saving of around £200 if you are still on a standard tariff."