Rail passengers could face higher fares and worse service following the decision to award the South West Trains (SWT) rail franchise to First Group, the competition watchdog has warned.
Concerns have been raised by the Competition and Markets Authority (CMA) about the London-Exeter route as First Group already operates Great Western Railway, which runs the only other train service between the cities.
The SWT franchise is currently operated by Stagecoach but will be taken over by First Group in partnership with Hong Kong-based company MTR on August 20.
CMA acting chief executive Andrea Coscelli said: "This is a crucial rail route to the South West, used by around half a million passengers a year. It's therefore vital that passengers do not suffer as a result of reduced competition.
"The CMA believes that without its intervention, First Group may be able to increase fares for passengers between London and Exeter, as it will be the only rail operator running all services on this route."
The firms involved have five working days to offer proposals to address the competition concerns.
If these are deemed insufficient the CMA can open an in-depth investigation.
On Monday Network Rail boss Mark Carne described the timing of the franchise takeover as "not ideal" as it falls in the middle of a three-week engineering programme at London Waterloo, the UK's busiest train station, which will cause a significant reduction in services.
But he insisted there is a "real determination to not allow this to be a distraction".