Network Rail has confirmed that there will be no train services between the two cities from Friday, April 3 to Monday, April 6, with rail replacement buses operating instead while essential maintenance is carried out in the Eastleigh area.
It has been described as a “vital corridor for passengers and freight”
The work will take place on the line between Winchester and St Denys, used by around 3,500 trains each week.
Engineers will be carrying out maintenance on switches and crossings — the movable sections of track that allow trains to change lines — as well as tamping and stoneblowing.
These processes involve lifting the track and adjusting the ballast underneath to ensure the railway remains correctly aligned and safe.
George Murrell, Route Renewals Director for Network Rail Wessex, said: “These improvements will provide better journeys for customers and a more reliable railway for freight heading to one of the country’s busiest and most important freight terminals in Southampton.”
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Changes to services mean trains between London Waterloo and Weymouth will be diverted via Guildford and Havant, adding extra time to journeys.
Services between London Waterloo and Portsmouth Harbour railway station via Eastleigh will terminate at Winchester, while trains between Salisbury and Southampton will not stop at stations via Eastleigh.
Rail replacement buses will run throughout the closure. A fast hourly bus will operate between Winchester and Southampton Central railway station, stopping at Southampton Airport Parkway railway station.
A stopping service will also run hourly via Shawford and Eastleigh. Additional buses will connect Romsey railway station and Southampton Central, as well as Winchester and Fareham railway station.
Passengers are being urged to check travel plans before setting out.