A "huge" landslip has caused a railway track to collapse, leaving passengers facing major disruption.
The 144ft-long (44m) landslip, which happened near Hook, in Hampshire, has damaged the main line from London Waterloo to Basingstoke.
Network Rail has asked passengers not to travel between London and south or west of Basingstoke on Monday.
It also said there would be "major changes to train services for some time" while repairs take place.
The slip, which left one track hanging in mid-air, means only two of the railway's four tracks are passable, with both the intact tracks designed to be used by London-bound trains only.
Network Rail Wessex route director Mark Killick said it would have "a massive effect on customers".
He said the main line to Basingstoke was the spine of the railway and there would be "knock-on impacts across the route".
"We're still assessing the damage and it's difficult to put a detailed timescale in place, but we know it's going to be at least a week," he said.
Apologising for the "scale of the disruption", he said passengers should check before they travel, not just on the affected section but all the way up the line to London Waterloo.
"We will need to stabilise the embankment, essentially stopping it moving, and then rebuild the railway where it has slid away," he said.
The slip happened when the soil gave way along a section of the embankment to the north-east of Hook station and slide out from underneath the tracks.
Network Rail said it was working on designs for the work needed to repair the railway, which would then give a clearer idea of timescales involved.
It added that a plan to provide journey options was set to be published "as soon as possible".
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