The Met Office said snow could hit the capital as early as 2pm today, and last at least seven hours.
The news comes as forecasters revealed the UK has seen its coldest temperature in six years, at a freezing -14.4C. The icy temperature was recorded in Braemar this morning.
Forecasters warned heavy snow is likely to fall between 2pm and 9pm today, and said there is a “danger to life” because of the warning.
The Met Office said: “A band of rain will arrive from the southwest on Thursday afternoon, quickly turning to snow and becoming heavy at times. 3 to 7 cm is likely to accumulate quickly – within two to three hours – with up to 10 cm in some places.
“The highest snowfall accumulations are likely to be in areas above 150 metres or so.”
Snow is predicted to hit London, the South East, the South West and Wales. The amber warning covers the edge of London and south-east England and most of south-west England and Wales.
As well as potential travel delays and cancellations, forecasters put out a warning about some rural communities being cut off because of adverse conditions.
The Met Office said: “There is a good chance that some rural communities could become cut off.
“Power cuts are likely and other services, such as mobile phone coverage, could be affected.”
The past two days have seen some services grind to a halt as schools were forced to close up and down the country.
The majority of schools that faced closing its doors in England were in Greater Manchester, Lancashire and Liverpool.
In Scotland, more than 40 schools were forced to close on Wednesday, with 30 shutting in Northern Ireland.
The runways at Manchester and Liverpool airports also had to temporarily shut on Wednesday.
On Thursday, concerned travellers from across the UK were seen to be making contact with airports to check the status of their flight.
Most airports have advised customers plan ahead and allow plenty of time to travel, including London’s Stansted Airport, but Bristol Airport told a customers it was “monitoring the situation and have snow clearing equipment ready”.
Those looking to travel by road have also been given guidance and warning ahead of snow.
Highways England released a “Severe Weather Alert” for Thursday and told those driving in the South West to check conditions before they travel.