The warning comes from the regional assembly and refers to new legislation to protect rail improvements in London.

The Crossrail project is expected to affect journeys into Reading and Paddington for about six years while construction is under way.

But the assembly claims new plans will leave little scope for expansion of South West services until 2045.

The need to work with the London project has been agreed by the First group which has just won the new greater western rail franchise that covers services in Devon and Cornwall.

The assembly says the bill must include capacity for growth outside the capital especially because South West rail travel is currently increasing between 5% and 10% each year.