Temperatures in the next few days will be up to 5C colder than expected at this time of year, with snow and sleet on the way, forecasters have warned.
The Met Office has issued yellow warnings for ice in parts of Wales, south-west England, Scotland and Northern Ireland.
London could see snow on Wednesday night and temperatures could drop as low as minus 3C (26.6F) in the Midlands.
There will be widespread frost over the weekend, with showers on Saturday and Sunday and 5cm to 6cm of snow in the Grampians.
Age UK has urged older people to take precautions in the cold weather to keep themselves "safe and well".
A spokeswoman for the Met Office said temperatures in the coming days will be lower than the average for the time of year.
"For example, in London it should be 2-3C above freezing at night and over the weekend it is expected to be minus 1C (30.2F)," she said.
"That is 4-5C below average."
Temperatures will start to "struggle" from Thursday, dropping in London from 7C (44.6F) on Wednesday to 4C (39.2F).
The North East coast, including Newcastle, will see temperatures of 3C (37.4F) with rain and sleet.
On Thursday night, some parts of the UK will be between minus 3C and minus 4C (26.6F and 24.8F).
Friday will also be cold, with widespread frost overnight into Saturday.
There may be snow on Sunday, with higher ground such as the Grampians seeing as much as 5cm to 6cm.
The Met Office spokeswoman said the colder spell was due to easterly winds from Scandinavia.
Age UK said cold weather can have a "devastating impact" on the health of older people, causing increased blood pressure and raising the risk of chest infections.
Caroline Abrahams, charity director at Age UK, said: "The cold weather can be really challenging for older people, particularly those who are more vulnerable because of pre-existing health conditions or who are living in housing that is difficult and expensive to heat.
"It can also be an especially lonely time, if older people feel it is too cold to go out and about.
"With the weather set to take a turn for the worse, we are urging people to check our advice to make sure they are doing everything they can to protect themselves against the cold weather.
"We'd also urge people to keep an eye on their older family members, friends or neighbours when the weather is particularly bad.
"Picking up some shopping for them or just popping in to check they're okay and having a friendly chat can be a real help at this time of year."