Homeowners will be at the mercy of high-pressure sales teams and unskilled retrofitters.
Hundreds of thousands more households could struggle to pay energy bills as a result of the coronavirus pandemic without more action to make homes warmer, it has been warned.
More than 200,000 households in England are set to fall into fuel poverty as the economy struggles to recover from the mismanaged coronavirus crisis, a coalition of experts, campaign groups and associations, local authorities and unions have warned.
With more people in cold homes they cannot afford to heat, putting them at greater risk of respiratory infections, a potential wave of Covid-19 in winter could be catastrophic for individuals and the health service, they said.
The End Fuel Poverty Coalition is warning of a "perfect storm" of cold homes, high winter fuel bills and Covid-19 hitting the NHS in winter.
The government announced £2 billion for improving the energy efficiency of homes in its recent summer economic statement, to help boost jobs across the country in the wake of the pandemic, cut bills and tackle climate change.
It also announced £1 billion for improving the energy efficiency of public buildings such as schools, hospitals and military bases.
But the coalition is urging ministers to go further to deliver on a manifesto pledge to invest £9.2 billion in building energy efficiency, extend the warm home discount paid to low income households, and introduce wider home upgrade grants.
Ministers must also take more steps to improve energy standards in the private rented sector, the groups urged.
There are already an estimated 2.4 million households in England in fuel poverty, facing low incomes and high fuel bills, often due to poorly insulated homes - and many of them are rented properties.
Estimates from the coalition, based on predicted rising unemployment and the proportion of homes where the breadwinner is unemployed that are in fuel poverty, suggest that could rise by more than 200,000 households.
Even more more families could be forced into fuel poverty in the face of both reduced income and higher bills from fewer working hours and spending more time at home, the coalition said.
William Baker, from Solutions to Tackle Energy Poverty and a member of the coalition, said: "It has never been more important for the Government to fix the roof while the sun is shining.
"While it is summer now, colder temperatures are on the way and hundreds of thousands more people will feel the harsh reality of fuel poverty.
"In just a few months we could see a perfect storm of cold homes, high winter fuel bills and a future wave of Covid-19 hitting the NHS during winter - a period when it always struggles to maintain services."
Ruth London from Fuel Poverty Action added: "The Chancellor's £2 billion for energy efficiency retrofits could have done wonders to relieve fuel poverty. Instead, targeted at homeowners and landlords, it will do very little for renters.
"And without clear protections, homeowners will be at the mercy of high-pressure sales teams and unskilled retrofitters, who can leave them colder, and poorer, than before."
A Business Department spokesperson said: "No one should be cold in their own home and that's why the Government acted quickly to secure an agreement with all energy suppliers in March to support customers impacted by Covid-19.
"We are protecting households from rip-off deals with our energy price cap, giving extra money to pensioners during colder times of year and providing £2 billion in funding for domestic energy efficiency measures."
Emily Beament is the PA environment correspondent.