Network Rail engineers will be working around the clock to install 25m-long soil steel nails that will be drilled into the slope and reinforced with concrete to support the cutting and protect the railway line.
Two years ago, Network Rail engineers invested £2.28m on a project to prevent landslips in the area that required the line to be closed for nine days.
During the closure, engineers excavated 5,000 tonnes of earth from the steepest part of the railway cutting near Fareham Tunnel, where the line is cut deeply into the hillside, and built an 80-metre-long retaining wall.
The present work follows the discovery of earth movement in a different part of the cutting. from the 2022 works, which have not been affected by the recent poor weather conditions, and no further earth movements detected in that area.
During the closure, South Western Railway services between Eastleigh and Fareham will be replaced by buses, except for Tuesday, 30 January when ASLEF members are on strike. London Waterloo to Portsmouth Harbour services via Basingstoke will be diverted via Netley and will not call at Fleet, Farnborough Main, or Woking.
Services may also be affected on Monday, 29 and from Wednesday, 31 January to Tuesday, 6 February because of an overtime ban by ASLEF.
Matt Pocock, Network Rail's Wessex route director, said: “There's never a good time to close the railway and I'm really sorry for the short notice of this closure, but the safety of our customers and railway colleagues is our number one priority.
“After the recent severe weather, our teams spotted land movement which means we have to act quickly to prevent a much worse and disruptive landslip.
“I would like to thank customers and local residents for their patience and understanding over the coming week while our teams complete this urgent work to stabilise the slope as quickly and safely as possible so we can get customers moving again.”
Stuart Meek, South Western Railway's chief operating officer, said: “We're sorry for the disruption that this short notice closure will cause, however, our colleagues at Network Rail must close the line between Eastleigh and Fareham to carry out safety-critical work.
“To run services between London Waterloo and Portsmouth Harbour via Basingstoke we'll need to divert trains from Fareham to St Denys, where they will stop and change direction before carrying on to London Waterloo.
“Due to the time this will take, Portsmouth Harbour to London Waterloo via Basingstoke services will be altered and will not call at Fleet, Farnborough Main or Woking. Customers travelling from or to these stations should check before travelling.
“We're already expecting short-notice alterations and cancellations across our network due to industrial action by the ASLEF union next week, so it's vital that customers check before travelling on these altered services.”