Forecasters say the freezing conditions will continue through this week
Millions of pensioners and vulnerable people around the UK have become eligible for cold weather payments.
The £25-a-week assistance is triggered when an area's average temperature falls or is forecast to fall to 0C or below for seven consecutive days.
Forecasts for the London area mean 600,000 people are eligible for fuel help for the first time in a decade.
As the Arctic snap continues to bite, the freeze has restricted water supplies in Rhondda Fach, South Wales.
Welsh Water apologised, saying the freezing conditions had "severely restricted" supplies feeding a local treatment works. There would be occasional loss of supply or discoloured water, but it was safe to drink, they said.
Tankers were bringing water in to the area, bowsers would be put in "key locations" and bottled water distributed to customers with special needs and mothers with young babies, added the firm.
Forecasters say temperatures will stay below zero in many parts of the UK.
Lows of -8C (17.6F) have been forecast for much of southern England and Wales, with temperatures dropping to -10C in rural Hampshire and Surrey overnight.
The mercury dropped to -11C in Aboyne, Aberdeenshire, and in Shap, Cumbria, on Monday night.
The £15m for Londoners means weather payments have topped £100m this winter.
Charities have warned that the elderly and sick urgently need financial help with heating bills to prevent avoidable deaths.
Britain's biggest pensioner group - the National Pensioners Convention - said 12 pensioners could die every hour during the cold snap.
On Tuesday, the Department for Work and Pensions said cold weather payments had now been triggered at 52 weather stations around the UK since the start of this winter.
With freezing conditions sweeping across the UK, payments are being made from Aviemore in Scotland to Bedford in southern England.
The payment, which goes to people in receipt of certain benefits - mainly pensioners, severely disabled people and families with a young or severely disabled child - rose this year from £8.50 to £25-a-week for each spell of cold weather.
It is paid automatically to those who qualify, including the estimated 2.7m households in receipt of pension credit.
Work and Pensions Secretary James Purnell said: "We don't want people to worry about turning up their heating when it's cold.
"That is why we've trebled the cold weather payment to £25, to put money in the pockets of the people who need it most.
"Millions of vulnerable people and pensioners are now in line for this extra help after the recent cold snap."
The increased cold weather payments are one part of a package of measures announced by the government this winter.
This year's Christmas bonus for pensioners and disabled people, which will be paid between January and March, has increased from £10 to £70.
The Met Office issued severe weather warnings on Monday night
And an extra £575m has been spent on winter fuel payments, with £250 for those in households with someone aged 60 or over, and £400 for those with someone aged 80 plus.
Meanwhile, Help the Aged has warned that the death rate rises by 1% to 2% for every temperature drop of 1C.
Older people and those weakened by illness are particularly susceptible to cold-related illness and death.
The Office for National Statistics said that from December 2007 to March 2008 there were an extra 25,300 deaths in England and Wales compared with the average for non-winter months - a 7% increase on the previous year's figure.
However, the figure was still some way short of the increase in deaths seen in the winters of the late 1990s when totals hit nearly 50,000 as flu swept Britain.
The National Pensioners Convention has called on the government to double the winter fuel allowance.
NPC general secretary Joe Harris said: "Up to three million pensioner households are currently spending more than 10% of their income on fuel bills and are living in fuel poverty.
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"Energy efficiency schemes won't help them pay their bills this month and neither will they prevent tens of thousands of pensioners dying from the cold this winter."
Macmillan Cancer Support is also calling on the government to extend the payments to those with cancer.
Chief executive Ciaran Devane said: "Cancer patients of all ages continue to suffer needlessly because of this dreadfully cold weather.
"They tell us they feel the cold more because of treatment and have to spend more time at home. And throughout this, their fuel bills soar whilst their household income has dropped."