No trains will run between Horsham and Dorking until Tuesday while ‘urgent repairs’ are carried out.
Network Rail announced the closure in an ‘urgent update’ on Twitter following a landslip earlier in the week.
A spokesman said: “Unfortunately, we have decided to shut the railway from the start of service tomorrow so we can get some urgent repairs done, which means both lines can reopen by Tuesday next week
“We need to install some sheet piling, which is a wall of steel vibrated into the ground to prevent further slippage.
“To do this work safely we need to close both lines, instead of the one line closure we have currently.”
He said the firm was ‘really sorry’ for the late notice of the decision.
The spokesman added: “This is the safest, quickest solution that will have the least impact on you. It also means we can have both lines reopened by Tuesday morning.
“This work is just a temporary fix though, so we can get the line reopen for you quickly.
“We will need to go back and complete a permanent fix working evenings and possibly weekends. Please continue to check before you travel with @SouthernRailUK.”