The forecaster issued its first country-wide Level 3 Cold Weather Alert of the winter - the second most serious.
The alerts are tied in to the Government's Cold Weather Plan and are relayed to organisations such as Age UK, which help the elderly through winter.
With temperatures this week as low as minus 6.2C in Pershore, Worcestershire, minus 6.1C in South Farnborough, Hampshire, and minus 5.7C in Hurn, Dorset, the Level 3 Alert is expected to remain in force until the weekend when the country is braced for snow.
The Met Office said: "This weather could increase the health risks to vulnerable patients and disrupt the delivery of services."
The only higher alert is Level 4, which sees normally healthy people at risk from the cold.
Britain's deep freeze has seen daytime temperatures plummet four or five degrees lower than average for February - traditionally the coldest month of the year.
Forecasters said Saturday will see a cold front sweep south and east across the UK from the west, resulting in snow where slightly milder air meets cold air.
The mercury - which hovered between 1C and 4C yesterday - is forecast to drop considerably today, with some areas remaining below freezing in the daytime and temperatures as low as minus 10C possible tonight.
Aisling Creevey, a forecaster with MeteoGroup, the weather division of the Press Association, said: "Today is set to be bitterly cold, with a few snow flurries in the east of England.
"There will be maximum temperatures of 3C and 4C but many places won't get above freezing.
"Overnight will be very cold, with temperatures down to minus 8C, maybe minus 9C and minus 10C in some places.
"As we go into Friday, we are looking at some snow showers affecting the eastern part of England, mainly in the south."
Ms Creevey said a 24-hour period from Saturday would see much of the country experience snow as a front moves in from the west.
She said: "Scotland could see between 5cm and 10cm of snow on Saturday, and there could be 2cm-5cm across England and Wales.
"But south west England is likely to see rain rather than snow."
Respite from the big freeze is not expected until Monday, when milder temperatures are expected, along with frost overnight.
While the sub-zero temperatures continue, a new shelter has opened for rough sleepers in the capital, funded by London Mayor Boris Johnson and run by homeless charity St Mungo's.
Plans are in place throughout the winter to ensure the 24-hour shelter in Lambeth is open and ready to receive people whenever three consecutive nights of freezing temperatures are predicted.
St Mungo's chief executive Charles Fraser said: "We supported more than 100 people into emergency shelter during last year's severe weather, which is the first step in talking to rough sleepers about the problems they are facing and what longer term support will help them recover."
The AA said it was 80% busier than normal and expected to attend more than 17,000 breakdowns today as the cold caused a surge in flat batteries.
Darron Burness, the AA's head of special operations, said: "As well as being a killer for car batteries, at those temperatures road salt is pretty much ineffective, so drivers will have to exercise a lot of caution, particularly later tonight and on the early-morning commute tomorrow."
Letters have been sent to 675,000 homes across England to raise awareness of home energy grants.
The mailshot concentrated on areas where there are high levels of fuel poverty, such as Birmingham, Leeds, Bradford, Sheffield and County Durham.
Pensioners and people on very low incomes can get money off having their heating system repaired or replaced, or their lofts insulated.
The scheme has helped 21,000 homes this financial year, but can help at least 16,000 more before the end of March.
Greg Barker, minister for energy and climate change, said: "The cold weather
we're experiencing at the moment reminds everyone how important it is to have a cosy home so I'm urging anyone who's eligible for a Warm Front
grant to pick up the phone.
"Getting a new boiler, fixing an old one or even just topping up loft insulation can lead to a warmer home in the future."
For further information call 0800 316 6004.
Send your snow pictures to websubmissions@independent.co.uk and we'll publish a reader's gallery of the best
PA
* It's a Mini disaster! BMW sponsors deadly cold front
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