Centrica, the owner of British Gas, has come under fire from fuel poverty campaigners before announcing profits of almost £3bn this week.

The energy group is expected to announce broadly flat group operating profits of £2.8bn for 2013, with the British Gas "UK downstream" business contributing £1.05bn, down 4% from 2012, City analysts say.

"A slight drop in already astronomical profits will provide little succour to the millions struggling with high bills, cold homes and unwanted prepayment meters precisely because of the profiteering of this company," said a spokeswoman for the Fuel Poverty Action group.

"Although Centrica care little for their customers freezing to death in their homes they have shown commitment to their shareholders, by recently handing back £420m to them, and a commitment to fracking in Lancashire, despite community objections, in their insatiable lust for profit," she added. Centrica, which has by far the largest market share of the UK gas and electricity supply market, has traditionally been a lightning rod for critics of the wider big six power providers.

A spokesman for the group insisted it was working hard to help tackle fuel poverty. "Last year we assisted nearly 2 million vulnerable households with discounts and free insulation. This winter more than half a million British Gas customers received the Warm Home Discount, worth £135. In addition, many of them also received a new top-up discount worth £60," said a spokesman.

Centrica's profit margin averages about 5% after tax across the group, he added: "We use this to make the big investments and commitments which secure vital energy supplies for British homes."

Centrica has spent more than £14bn on a series of big gas contracts with companies in the US and Qatar under which it buys wholesale supplies that can last 20 years. But the company has also thrown its weight behind attempts to prove shale gas reserves in Britain through a partnership with Cuadrilla Resources under which it takes a 25% stake in a promising Lancashire exploration licence.

Centrica's British Gas featured prominently in a Which? review of complaints about the big six from customers. The worst offender was npower but 24 of every 1,000 British Gas customers issued complaints against the company in the last three months of 2013 after domestic price rises had been announced.

Shares in Centrica fell 3% last Monday after a warning by Ed Davey, the energy and climate change secretary, that large power firms might need to be broken up to tackle market dominance.

The company argued that Britain had one of the most competitive energy markets in the world, with gas and electricity prices among the lowest in western Europe.

But Fuel Poverty Action said it wanted a not-for-profit, renewable energy system which put communities and those in fuel poverty first.

Until that happened, said the spokeswoman, "the likes of British Gas will continue to make a killing and in return will remain a target for anger and protest".