Three million elderly people fear they will not be able to stay warm in their own homes this winter, following the recent steep increases in the cost of heating, according to research published today.

The plight of many older householders emerged as the Government faced renewed calls to offer immediate help to lower-income families struggling to pay energy bills. Four of the “Big Six” energy companies have raised their prices before the winter surge in demand, with the average combined electricity and gas bill now standing at £1,267 per year.

Executives from the firms, which have been accused of acting as a cartel, will appear before MPs tomorrow to defend the sharp rises. Yesterday their trade organisation dismissed calls for a windfall tax on the Big Six, insisting their profits were not “particularly big”.

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Over the weekend it also emerged that energy companies have been using tax loopholes. Although he declined to comment on individual companies, Danny Alexander, the Chief Secretary to the Treasury, said yesterday: “People are rightly livid about companies and individuals avoiding paying the proper amount of tax. I’m livid about that. It is something which is not acceptable at any time, but particularly at a time when we’re going through tough spending choices.”

He was speaking after The Independent on Sunday reported that three companies – Scotia Gas, UK Power Networks and Electricity Northwest – had saved £140m between them by using legal tax loopholes to minimise their liabilities. With gas and electricity prices continuing to dominate exchanges between political leaders, a survey for Age UK found that 28 per cent of pensioners said their main concern for the coming cold months was ensuring they could heat their homes. The charity said the figures suggested the problems could affect as many as three million older people across the UK.

Age UK also raised the alarm over the health dangers to the elderly people, warning that cold weather and poorly heated homes increased the risk not only of influenza but also of heart attack and stroke. There are about 24,000 excess deaths in a typical British winter, many of them preventable.

Age UK said more than 40 per cent were caused by heart attack or stroke. Caroline Abrahams, the charity’s director, said: “It’s vital for older people to keep warm, both inside and outside their homes in the winter months. Being cold, even for just a short amount of time, can be very dangerous, as it increases the risk of associated health problems and preventable deaths during the winter.”

Senior executives from the Big Six will be challenged to justify the recent price hikes when they appear before the Commons Energy and Climate Change Select Committee. Simon Hughes, the Deputy Liberal Democrat leader, said Chancellor George Osborne should use the Autumn Statement in December to announce emergency help for families struggling with bills.

“I would like people to have a rebate on energy bills that would help the poorest most and would mean that there would be immediate relief this year, not waiting for the post-election period,” he told BBC1’s Sunday Politics.

Ed Miliband, the Labour leader, has said an incoming Labour government would force energy firms to freeze their prices for 19 months, while the former Tory Prime Minister Sir John Major has called for a levy on their profits.

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But Angela Knight, chief executive of Energy UK, which represents the companies, said their profits were not large enough to justify a windfall tax. “The profits here are, what, four to five per cent, four or five pence in the pound. That isn’t particularly big,” she said. She added that the companies were also making large investments in the UK and therefore had to have an “operating margin”.

The Government has invested an extra £500m in A&E services in a bid to avoid another winter crisis on emergency wards. A&E units have been under increased pressure for several months. MPs warned in the summer that the system may struggle to cope in the event of a major winter flu pandemic.

Dr Paul Cosford, director for health protection and medical director at Public Health England (PHE), said: “In colder weather, keeping yourself warm is essential to staying healthy, especially for the very young, older people or those with a chronic condition such as heart disease and asthma. There are a range of health problems associated with cold housing and winter weather, but, in particular, a cold indoor or outdoor environment can make heart and respiratory problems worse and can be fatal.”

PHE said living-room temperatures should ideally be kept at 70F (21C) and above, whereas bedroom temperatures should be kept at a minimum of 64F (18C).

Health leaders have also urged all at-risk groups – including the over-65s – to have a flu vaccination.

PHE will work with the Met Office between 1 November and 31 March 2014. Low temperatures of 2C or less or a spell of heavy snow will trigger cold weather alerts, which require hospitals, social care systems and GP surgeries to ensure they are prepared for spikes in demand.