Southern Railway has suspended services along parts of the South Coast due to 'extreme weather'.
The company says it is 'unable' to provide a service between Fareham and Southampton Central - and has cancelled trains from midday until 9pm tonight.
Southern trains don't stop at every station on this particular route like other operators, so the provider said the service will start and terminate at locations before the restrictions.
It means trains will run at slow speeds for a long distance. Rail passengers are being warned their journeys could be delayed by at least an hour.
Trains to and from Portsmouth Harbour are unaffected, but London Victoria to Southampton Central services, in both directions, will run to and from Bognor Regis only.
Passengers can use their tickets to travel on South Western Railway and Great western Railway services at no extra cost.They can also be used on London Underground between London Waterloo, London Bridge and London Victoria on the following routes:
If you're travelling from London, change at Barnham for a service to Havant (towards Portsmouth Harbour), changing once more for SWR services towards Southampton Central
If you're travelling from Southampton Central, change at Havant for a Southern service to Barnham, changing once more for a service towards London
You can also travel directly to London Waterloo from Southampton Central if you prefer, using TfL tube services to move about between London Waterloo and London Victoria
Brighton to Southampton Central services, in both directions, will run to and from Fareham only
If you're travelling from Brighton, change at Fareham for SWR services to Swanwick and Southampton Central. If you're going direct to Southampton, you can also use GWR
If you're travelling from Southampton Central and need Swanwick only, please travel with SWR only. If you're travelling to Fareham or further, you can use SWR or GWR services
A spokesperson for Southern Railway said: "On the railway, hot summers can be as challenging as freezing winters.
"Hot weather can affect the rails, overhead power lines and the ground on which the track sits on.
"On hot days like today, we plan ahead to prevent incidents which may cause significant disruption to your journey."Passengers may be entitled to compensation if they experience delays.
People are urged to keep train tickets and make a note of their journey to support any claim.