The large landslip near Harbury Tunnel is to keep the railway line between Banbury and Leamington closed for several weeks, Network Rail has said.
Specialist engineers are currently assessing the landslide – which is still slowly moving – to determine the scale of the problem.
Remote sensing techniques are being used to monitor the landslide and remedial works will begin as soon as it is safe to do so.
Mark Carne, chief executive of Network Rail, said: “This is a massive incident and I am making sure we have all the resources we need to tackle this problem.
“Our safety precautions ensured the railway was shut before there was any risk to passengers.
“Looking forward, our focus is on making sure the site is safe before our engineers move any of the affected earthworks.
“We know just how important this key route between Birmingham and London is, for both passengers and freight.
“This is a very deep cutting built back in Brunel’s days which we have been repairing for some time.
“I can assure passengers whose journeys are being disrupted that we will do our utmost to restore their services as soon as possible.
“However, it may take several weeks to stabilise it before we can start some services.”
Network Rail will be working with the train operators on alternative services for the next few weeks, and will be able to give a clearer idea on timings within the next two weeks.
The cutting forms the approaches to Harbury Tunnel, and is excavated to depths of up to 30 metres through inter layered mudstones and limestone.
Network Rail been carrying out stabilisation work at the site after a much smaller landslide elsewhere at the site on 14 February 2014. These works were underway when the slope failed at 14:30 on Saturday 31 January. No one was injured when the landslide happened.
The company is working with landowners to gain access and plan the safe removal of materials – with initial investigations suggesting that up to 350,000 tonnes of earth and rock will need to be removed.
The line carries long-distance services for Chiltern Railways and CrossCounty Trains, as well as substantial freight services.
Bus replacement services are running between Leamington and Banbury and a revised service is in effect between Banbury and Marylebone, Birmingham and Leamington , and Stratford and Leamington.
Passengers using Chiltern Railway and CrossCountry services are asked to check the National Rail website before travelling at www.nationalrail.co.uk