Consumer groups and politicians last night demanded swift action to help thousands of households threatened with fuel poverty this winter after two energy companies became the latest to announce massive increases in gas and fuel bills.

The rises by Npower and ScottishPower, which have 11.8 million customers between them, follow some of the biggest price increases during the past five weeks by the rest of the "big six" power companies. Npower said prices for gas customers would increase by an average 26%, with prices for electricity customers up by 14%, both with immediate effect. ScottishPower followed a few hours later by announcing it would increase gas prices by an average 34% and electricity prices by 9% from Monday.

Energywatch, the consumer watchdog, said average energy bills had gone up 38% in 2008, setting a new record for price rises since the energy market was opened up a decade ago.

As well as facing discontent from voters already suffering soaring food and petrol prices, the prospect of Gordon Brown facing an immediate political crisis intensified as a result of yesterday's announcements.

Frank Field, the Labour MP who led the back-bench rebellion over the 10p rate of income tax, said he would go "right to the line" to help people suffering fuel poverty.

It comes just days after it emerged more than 70 Labour back benchers have signed a petition calling for the introduction of a windfall tax on energy companies to help hard-pressed families - a move that ministers are thought to be anxious to avoid for fear that it will merely be passed on to consumers and will damage the industry's commitment to alternative energy sources.

Both Npower and ScottishPower blamed the soaring wholesale costs, saying they had made current pricing levels unsustainable.

They are the last of the country's suppliers to hit households with increases in recent weeks, following moves by British Gas, EDF, E.On and Scottish & Southern Energy.

Npower said the average gas customer would see an average annual increase of £162 with electricity bills rising by an average £60. The firm claimed that until yesterday its domestic gas prices had been the same as they were 18 months ago following a price decrease in 2007 and an increase in January, but wholesale costs had doubled.

Electricity generation had also been directly affected by the soaring price of raw materials, which had increased by 122% for gas and coal and 79% for oil over the last 12 months.

ScottishPower said the average dual fuel customer would see bills increasing by 25%. About 1.2 million of the company's customers were currently protected by fixed-price tariffs, it said.

There were calls last night for action ahead of a forthcoming report by Ofgem, which regulates the energy market, into allegations of uncompetitive practice and the effects of price rises on vulnerable customers.

Adam Scorer, director of campaigns at Energywatch, said: "Six price rises from the Big Six' in just five weeks caps a miserable summer for consumers and highlights just how the companies appear to act in step with each other - even if not in concert.

"The damaging analysis of the Commons Business and Enterprise Select Committee is reinforced week by week. The market clearly needs urgent reform. Ofgem's probe into the market, on which much will depend, is due to report in September. It must be a trigger to action to restore competition."

The fuel poverty charity National Energy Action (NEA) estimated that the latest round of price rises would push one in five UK households into fuel poverty.

NEA chief executive Jenny Saunders said: "It is imperative that the government announce a comprehensive package of measures to help people who face a desperate struggle to pay fuel bills."

A Department for Business, Enterprise and Regulatory Reform spokeswoman said: "It is clear the era of cheap energy is over."