Some people on the lowest incomes will receive a £25 cold weather payment due to the freezing conditions.
The Met Office has warned of an Arctic blast hitting the UK, which could cause overnight temperatures as low as -10C by the end of the week.
Motorists could face treacherous conditions on the roads, with train journeys taking longer than usual.
The winter payments were triggered for eligible households where the average temperature has been or is forecast to be 0C or below over a period of seven consecutive days.
According to new research, more than three million households cannot afford to heat their homes during the current spell of cold weather, putting their health at risk.
The UK Health and Security Agency has issued a Level 3 cold weather alert - and says vulnerable people should heat their homes to at least 18C, wear extra layers of clothing, and eat hot food to protect themselves.
But about 710,000 households cannot afford to follow this advice because they cannot pay for warm clothing, heating and food - and another 2.5 million families on low incomes are going without.
The research was carried out by the Joseph Rowntree Foundation, and its senior economist Rachelle Earwaker said: "The dangerously cold weather on the horizon is cause for concern.
"People are being forced to wager their financial health and whether they can afford more debt, against their wellbeing without sufficient heat, clothing or hot food."
The survey of 4,251 people in the bottom 40% of incomes suggested that about 4.3 million have cut the amount they spend on heating.
It also found that many families are already behind on their bills, owing more than £1,600 on average. And temperatures are likely to remain low for some time yet.
'Arctic maritime airmass': Snow and ice forecast
The Met Office has extended weather warnings through to Saturday, with one still in place for northern Scotland on Sunday.
Large parts of western and northern England, Wales, Scotland and Northern Ireland will be hit by snow and ice on Friday and Saturday, with weather warnings in place.
At least five centimetres of snow was confirmed in Aboyne, Aberdeenshire and Altnaharra in the Highlands on Friday morning, with three centimetres falling in Dyce near Aberdeen.
Schools were closed due to freezing conditions in parts of Aberdeenshire and the Highlands, while snow was seen on the ground in Edinburgh.
Met Office chief meteorologist Steve Willington warned there is an "increasing risk of snow as the week progresses".
"As an Arctic maritime air mass settles across the UK, temperatures will fall with widespread overnight frosts, severe in places, and daytime temperatures only a few degrees above freezing," he said.
"However, the cold air from the Arctic will also bring brighter conditions, with some dry, sunny spells, particularly away from the coast and where winds are light it could feel pleasant in the sunshine. Some patchy freezing fog is also likely."
Mountaineering Scotland's senior mountain safety adviser, Ross Cadie warned people in Scotland not to risk getting into difficulty out walking.
"When winter arrives in Scotland's mountains, we need to make sure we do our homework before heading out," he said.
"Planning and preparation from trusted sources and matching your adventure to your level of skill and conditions will help you return home safely."
Beware of slippery roads and pavements
Roads, pavements and cycle lanes could all be slippery, the Met Office said, and motorists have also been warned to keep blankets in their vehicles in case they break down.
RAC spokesman Rod Dennis added: "Our advice is to be winter ready - check tyres are properly inflated and with good tread, while topping up oil, coolant and screen wash levels if needed.
"Drivers with older batteries in their cars might also wish to give their vehicle a 20-minute drive before colder conditions arrive to ensure the battery can cope with sub-zero temperatures.
"It's also worth having a fully charged mobile phone and carrying a blanket in case of a breakdown to keep warm."
No gas for heating or cooking for five days
Meanwhile in the Sheffield area of Stannington around 1,080 homes have had no gas since a water main burst on Friday night - sending hundreds of thousands of litres of water into the gas network.
At a press conference on Thursday morning, it was announced 400 customers had gas restored overnight, with people in Malin Bridge still waiting for water to be cleared from the system until their gas can be reconnected.
Cadent, the firm which runs the gas network, says 150 people are working day and night to fix the problem, and they have started to reconnect gas to some homes. Yorkshire Water said it is not yet clear who is at fault.
Sheffield City Council has declared a major incident so that staff can be redeployed to the area.
Asked about the approaching cold snap, leader Terry Fox said: "We're very worried... but, what we've seen, to be brutally frank, is a real deep community spirit where people are helping individuals."
Sheffield Hallam's Labour MP, Olivia Blake, has said she has asked Chancellor Jeremy Hunt for emergency funding for the council and raised the issue in the Commons on Wednesday, but said she has not yet received a response.