As a result, the Big Six suppliers have seen their profit margins soar to a five-year high.

They are now making an average profit of more than £105 a year from every dual-fuel customer, according to the industry regulator Ofgem. The figure has increased by 40 per cent in just three months.

It means energy suppliers have been raking in record multi-billion pound profits from families forced to turn up their heating.

At the same time vulnerable old people have been putting their health at risk by keeping their homes cold because they are worried about running up the bills.

The scale of the rip-off will emerge later this week when British Gas is expected to reveal an increase of more than 50% in its profits, taking the figure over £550m.

The company recently cut the price of gas by 7%, apparently in an attempt to head off criticism of its profits, but no other supplier has followed suit.

Four other members of the Big Six – Npower, Eon, EDF and Scottish Power – are foreign-owned, so their UK profits can be buried in the accounts of their parent companies. Consumers will be left in the dark about how much they are being ripped off.

Climate Change Secretary Ed Miliband said last night: ‘Ofgem’s report shows that energy companies need to cut their prices. British Gas’s recent cut was a welcome first move, but we need to see all suppliers passing on the full benefits of lower wholesale prices.’