People are facing "dreadful consequences", a charity has warned, as parts of the UK face plummeting temperatures.
The Met Office has issued several yellow weather warnings for snow and ice over the next few days, and the UK Health Security Agency has also issued a level three cold weather alert covering England.
National Energy Action said more government support is needed for "those at greatest peril" in the below-freezing conditions.
Adam Scorer, the charity's chief executive, said: "Impossibly high prices and now cold weather will leave millions struggling to stay warm and safe at home.
"Our figures show that 6.7 million UK households are fuel poor after energy prices have almost doubled in a year.
"We hear daily from people who are forced to turn their heating off when they need it the most.
"The vicious choice is either huge debt or an unheated home, with dreadful consequences either way.
"We will now start to see just how bleak this winter is going to be."
The Met Office has issued a yellow warning for ice on Friday covering northern England, England's western and eastern coasts, the Welsh coast, and the northern part of Northern Ireland.
There is a yellow warning for ice and snow in most of Scotland, which continues on Saturday and Sunday.
Many schools in the Aberdeenshire area have already been forced to close, while some travel disruption was also expected causing longer journey times.
Also on Saturday, there is a yellow warning for ice for the north of Northern Ireland and the western side of the UK, including the Welsh coast and Cornwall.
Temperatures were expected to fall as low as -10C overnight into Friday, and roads, cycle paths and pavements could be icy, the Met Office said.
The low temperatures triggered cold winter weather payments for some postcodes in England and Wales.
This means that people on low incomes receive a £25 payment if the average temperature in their area has been (or is forecast to be) 0C or below over seven consecutive days.
The Department for Work and Pensions said these payments could be triggered through to the end of March.
A spokesman added: "We're committed to protecting the most vulnerable in our society and this additional help comes on top of wider Government support including £1,200 in direct payments already issued to millions of low-income households this year.
"Alongside this, we're providing households with £400 towards their energy bills this winter, with our energy price guarantee saving the typical household another £900 on top of this."
Meanwhile, The Daily Telegraph has reported a warning from the National Grid that electricity supplies will be tight on Friday and Sunday due to the cold weather.
The grid said it might need to use "enhanced actions", which the Telegraph said include a new scheme where households are paid to use less electricity to ease pressure on supply.