Fresh snowfalls expected overnight in south-east England could leave parts of Kent, Sussex and the Essex coast with up to 15cm (6in) of snow by Thursday.
Snow fell across the South East, including on Brighton beach
Gritter lorries have been out in force as forecasters predict the UK's worst winter weather so far.
Snow and ice led to travel problems in the South East and London on Wednesday, with rail services severely disrupted and heavy congestion on some roads.
The Highways Agency has advised drivers to take extra care on the roads.
BBC weather forecaster Phil Avery said accumulations of up to 10cm (3.9in) were possible over the Downs on Wednesday night.
Snow storms hit some northern areas on Tuesday
Some snow flurries could also reach east London, while a small weather feature would bring some snow across the Midlands, central southern England and parts of the West Country.
An average of 2cm (0.8in) of snow fell across southern and eastern England overnight into Wednesday, bringing an end to the mild weather.
Rail passengers suffered delays when points failed and hi-tech systems designed to protect trains from the effects of ice on power rails left them "stuck" and unable to move.
An estimated 80,000 South West Trains users, a further 60,000 Southern customers and up to 200,000 Southeastern passengers were caught up in the morning rush hour disruption.
Several thousand passengers on One services between London and East Anglia were also affected.
A spokesman for Network Rail said it had done everything it could to minimise the disruption in the face of the "extreme weather".
London Underground commuters suffered delays and cancellations on six lines, while 21 flights had to be cancelled from Heathrow Airport, and three from Gatwick.
Norwich Airport also suffered some early delays.
On the roads, a jack-knifed lorry closed part of the M25 in Surrey, with traffic tailing back from junction nine to junction five in Kent.
The congestion was added to by a crash on the northbound M23 in Surrey, which meant the motorway was closed in the direction of the A23 with all traffic diverted onto the M25.
In East Sussex, a car skidded into a gas main in Buxted, leading to the evacuation of a number of homes and commercial premises.
Twenty-three schools across the South East, most of them in Sussex, were closed due to treacherous driving conditions on ungritted access roads.
Kent, East Sussex and West Sussex county councils deployed a total of 113 gritting vehicles on main routes.
Surrey County Council said its fleet had been spreading "wet salt", a new environmentally friendly saline solution.