The expiring deals will collectively add £28m to the energy bills of fixed term customers, who will be automatically rolled onto their supplier’s default tariffs.

The average hike will be £140 per year, but households with Together Energy and Peterborough Energy face the biggest increases of £302 and £258 respectively, according to Uswitch.

Of the Big Six suppliers, ScottishPower and British Gas customers will be hit hardest, with bill increases of £210 and £133.

Default or standard tariffs tend to be the most expensive, and although the energy regulator introduced a new price cap at the beginning of the year to try and give millions of households on these tariffs a fairer deal, the cap only sets an upper limit for a supplier’s default tariff.

The cap is £1,137 per year for the typical customer, which is still £165 more expensive than the cheapest fixed deal on the market, Uswitch said.

Double whammy

In a further blow to customers, Ofgem is expected to raise the level of the energy price cap by £80-£100 on 7 February, effective from April, meaning people with deals ending this month face a double whammy if they don’t take action.

Rik Smith, energy expert at uSwitch.com, said: “Around 200,000 households will see their energy deal end this month and with the threat of price hikes around the corner they are set for a double blow unless they take action.

“Those affected should have received a letter over the Christmas period telling them their bills will go up, but understandably there are other distractions at that time of year. However, it’s vital they now take action and lock in a new fixed deal. Not only will they save money straight away, they’ll also protect themselves from the inevitable wave of price rises that will hit on 1 April.”

Ofgem rules allow customers to switch suppliers without paying exit fees 49 days before their plan end date, so households affected can switch and save now without worrying about charges for leaving early.