An alliance including the charity Barnardo's says the UK is second only to Estonia in Europe for the number of people struggling to pay their heating bills.
Yesterday, ScottishPower announced its prices would rise by an average of between 8.5% and 9%, adding £113 to the typical bill.
The company is now the fourth of the "big six" energy firms to unveil similar hikes in recent weeks. In part it blamed so-called green taxes for the rise.
Deputy First Minister Nicola Sturgeon said it was "another blow to Scottish households, many of which are already struggling to pay their energy bills".
Labour MP for Glasgow North West John Robertson said the hike was morally wrong and would force some customers to choose between eating and heating their homes. He accused the firm of treating consumers "with contempt and ripping them off".
Fuel poverty alliance the Energy Bill Revolution has written to Prime Minister David Cameron today demanding action from politicians.
It warns "woeful" levels of insulation have led to homes falling way behind those of comparable European countries.
It said this was pushing up bills, despite the low wholesale cost of gas in the UK compared to most European countries.
More than five million UK households are thought to live in fuel poverty, spending more than 10% of their income on energy.
The Energy Bill Revolution has called for a programme of home insulation, which it said could save families up to £500 a year.
Campaign director Ed Matthew said: "The biggest chance to cut energy bills is to fully insulate the UK's leaky homes. No other investment can do so much for so many.
"If the (Coalition) is serious about solving this crisis it must make insulating homes the number one infrastructure priority."
Earlier this week Mr Cameron announced plans to roll back green taxes in a bid to lower bills.
But yesterday Deputy Prime Minister Nick Clegg made clear he would not accept any move that scrapped a system he said helped "two million of the poorest households in this country".
However, he suggested that the taxpayer could pick up the bill.
He said: "We will provide £135 to two million low income households, which is funded from one of these levies. Now, of course, we could perhaps fund that through general government expenditure rather than one of those levies.
"We do need to help those two million low-income families with their fuel bills this winter."
Labour leader Ed Miliband described this as a "panicked wheeze paid for by taxpayers".
Speaking on a visit to Brussels, Mr Cameron reiterated his pledge to look into whether government levies were inflating prices.
He said: "I said yesterday we've also got to look at the taxes and charges and things that are added to people's bills to see if we can reduce the cost of those. "
Iberdrola, the Spanish firm which owns ScottishPower, said it had "no option" but to increase prices and blamed in part the Coalition's energy efficiency and environmental measures.
It comes days after ScottishPower agreed to pay out £8.5 million to customers for mis-selling products following a probe by regulator Ofgem.