Correction: the original version of this story said the closure will happen in January. GTR has now clarified it is actually February.
The London to Brighton line will close for nine days between Brighton and Three Bridges in February so the track can be replaced.
The work is being carried out at Copyhold Junction, near Haywards Heath, between Saturday, February 19 and Sunday, February 27.
The line will also be closed from Burgess Hill to Lewes and a new underpass built under the railway at Hassocks while the line is closed.
When lines are closed, alternative travel arrangements will be put in place for passengers, similar to the February 2019 Three Bridges to Brighton closure, including replacement buses and ticket acceptance on open routes.
More work will be brought into the project over the coming months.
The works will involve more than 7,000 tonnes of ballast, more than 1km of track renewal and eight sets of point renewals.
Network Rail Southern region’s investment director Paul Harwood said: “We never take the decision to close the railway lightly and have reviewed this work and access approach in light of the difficult year for passengers, the industry and the country at large, to confirm it is the best option.
“As we need to get this critical work done quickly and efficiently to help the country and the railway build back from the pandemic, we believe that getting the work done in 9 days and the supporting weekends has never been more important.
“The alternative to delivering this investment over the extended closure in winter, when demand is naturally lower, would have been up to 20 weekend closures or multiple bank holiday closures over two years, which we are confident is unacceptable to our neighbours, passengers and stakeholders, in particular the leisure and tourism industry on the south coast.
“We are working closely with Govia Thameslink Railway to plan the alternative travel arrangements and make passengers aware of the impact on their journeys, and will regularly liaise with stakeholders and passenger groups as the plans develop.”
Chris Fowler, customer services director for Southern, said: “This route is one of the country’s busiest, and Network Rail’s track renewal is essential to give our customers the reliable, on-time services they rightly expect. We’ll be ensuring that alternative transport is in place, and we’ll issue detailed travel advice over the coming months.
“Journeys will take considerably longer during the nine days of work and we thank customers in advance for their patience.”