A signalling problem means fewer trains will be running between Yeovil Junction and Axminster.
This is the Waterloo to Exeter line which passes through north Dorset and east Devon.
The problem is due to an earlier power failure at Crewkerne, affecting South Western Railway trains.
Services running through these stations may be cancelled, delayed or revised. Disruption is expected until the end of the day.
A spokesperson for South Western Railway said: "We have been informed of a signalling problem between Yeovil Junction and Axminster. This means that only one train is allowed in the area at a time which may lead to delays and alterations.
"As a result of this, fewer trains between Waterloo and Exeter will run beyond Yeovil Junction so please check before you travel."
Tickets will be accepted at no extra cost on the:
- London Underground between Waterloo and Paddington via any reasonable route
- Great Western Railway services between London Paddington and Exeter St Davids
- Stagecoach South West 1 between Exeter - Pinhoe - Broadclyst (near Cranbrook)
- Stagecoach South West 4 between Exeter St Davids and Cranbrook
- Stagecoach South West 44A between Axminster - Honiton - Cranbrook - Exeter
The spokesperson added: "For further information or onward travel advice please speak to a member of staff or use a station help point.
"We are very sorry for any delay that this may cause to your journey."
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