Train services between London Waterloo and Devon have been halved by South Western Railway (SWR) due to unusually dry soil conditions along the route.
SWR said it had "no alternative" but to cut the services.
From 11 August, until further notice, there will be just one train every two hours between London Waterloo and Exeter St Davids, down from one per hour.
One train per hour will operate from London Waterloo to Yeovil Junction, Somerset, with journey times extended by about 25 minutes.
The disruption stems from a lack of moisture in clay soil embankments, which has disturbed track levels and made it unsafe for trains to operate at full speed.
This follows England’s second driest spring on record, with summer rainfall across the UK also registering just below average.
SWR said that the parched conditions had caused embankments between Gillingham in Dorset and Axminster in Devon to shrink.
This shrinkage has disturbed the overlying track, necessitating a speed limit reduction from 85mph to 40mph across a 12-mile section of the line.
SWR warned that dry conditions are “set to continue”, so further speed restrictions “may be required”.
Attempting to stabilise the track now would “only provide a short-lived fix as the soil continues to move”, it added.
SWR chief operating officer Stuart Meek said: “We are very sorry for the disruption that customers will experience due to this change, as we know just how important the West of England line is to the communities it serves.
“We have not taken this decision lightly and we recognise the impact that it will have on customers west of Yeovil in particular.
“However, to continue operating a safe and reliable service, we have no alternative but to introduce a reduced timetable.
“We always aim to minimise disruption and so, together with our colleagues at Network Rail, we will carefully monitor train performance and work hard to give customers a service that they can rely on.
“We encourage customers to plan ahead via our website and allow extra time to complete their journeys.”
Network Rail operations director Tom Desmond said: “The safety of our customers is our number one priority, which is why we must impose these speed restrictions between Gillingham and Axminster.
“We want to give customers the longest notice possible of timetable changes, and following a comprehensive analysis of ground and track conditions, we know that we must make changes in the next two weeks.
“We have worked closely with our colleagues at South Western Railway to understand the impact of the speed restrictions, and we will regularly review conditions in order to restore the normal timetable as soon as possible.”
The amended timetable will restore services to Crewkerne, Somerset which has been served by buses since 28 July because of dried out embankments.
SWR became the first operator brought into public ownership by the Labour Government in May.
