Rail passengers travelling through Winchester are being warned of disruption that could last all day because of emergency engineering work.
Repairs are required on track between Basingstoke and Winchester.
Engineers are expected to be at the site, reportedly in the Micheldever area, all day.
South Western Railway says that most trains between London Waterloo and the Poole or Weymouth will be diverted via Havant.
A replacement bus service will also be running between Basingstoke and Southampton Central.
CrossCountry services have also been affected. The rail companies are urging all passengers to check before travelling.