Energy suppliers are seeking to tie customers to fixed deals costing as much as £4,000 a year, as ministers face growing warnings over “untenable” proposed rises to the price cap this spring.

A 12-month fixed deal for a typical household now costs an average of almost £2,500, according to data from comparison website uSwitch.

That is £500 more per year than the £2,000 level that Ofgem, the energy regulator, is expected to increase the cap on variable tariffs to in April.

Ovo Energy, the UK’s second-biggest supplier, is offering a fixed-rate deal worth just under £4,200, according to the data.

Martin Young, an energy analyst at Investec, said the figure was “staggering” but has predicted energy prices will stay elevated or rise even further next year. He said: “Between the end of November through to the last week, fixed price deals have jumped up.”

The figures underline the stark difference between the costs of providing energy and how much suppliers can charge under the cap on bills. In recent months, the cap has lagged behind surging gas prices, shielding consumers from huge price hikes but forcing suppliers to sell energy at a loss.

It is expected to rise from £1,277 to £2,000 in the spring, but if fixed deals remain pricier – and families choose to stay on variable tariffs protected by the cap – Mr Young warned that suppliers could be put under even more financial pressure. The cap is currently updated every six months, meaning it is “for all intents and purposes a six-month fixed deal” that consumers can leave at any time. This means suppliers could be left in the lurch if they buy energy in advance for variable tariff customers only for them to leave when cheaper fixed deals become available.