Rail services in parts of Surrey, Sussex and Kent are the worst in the country.
Official figures have revealed almost one in five Southern trains ran late last month according to Network Rail.
South West Trains did much better with 91% of services on time.
First Great Western and Southeastern saw 88% to schedule.
Top performing were C2C in Essex at 97%. Chiltern in Oxfordshire and Buckinghamshire were 93%.
£6.5b pounds is being spent improving rail services for passengers on Thameslink in Sussex and Surrey. The Government has unveiled new trains that will run on the line - one of the busiest in Europe. Ria Chatterjee reports.
Southeastern and Southern rail services are expected to face some disruption tomorrow, according to the train operators.
Southern is introducing a special timetable which means there will be fewer services on some routes.
Southeastern is today experiencing some snow related problems but all lines are running.
Tomorrow the company is planning to cancel a number of early morning services to enable longer trains to run which reduces the possibility of failures.
An army of Southeastern staff volunteers are out at stations in Kent and Sussex to try to clear them of ice and snow.
The company says many workers have turned up at stations to carry out the work in an attempt to reduce the risk of injury for passengers.
Full details will be on the companies' websites later.
South West Trains and First Great Western will be operating as many services as possible but passengers are advised to check later tonight for any potential problems.
Chiltern Railways says significant snowfall is expected over the next 24 hours.
A spokesman said: "We will be running our regular timetable, but there is a risk we may experience delays and short notice cancellations. Please check before travelling."
Advance Warning for Monday 21st January 2013 - No service on South West Trains between Ascot and Ash Vale, between Virginia Water and Weybridge, between Salisbury and Bristol Temple Meads and between Guildford and Leatherhead due to poor weather conditions.
All other routes are open, but there is no train service at Addlestone, Bagshot, Beaulieu Road, Camberley, Chertsey, Frimley, Longcross and Upper Halliford.