Work to upgrade railway signalling equipment between Lewes and Seaford, which has caused the line to be closed for four days, will not be completed.
Network Rail has conceded it would not have enough time to complete the project on deadline.
But the four-day closure is going ahead, starting today (Thursday, March 7) and continuing until Sunday.
Paul Harwood, director of investment, South East, said: “Following discussions with our contractors over the past week, we’ve come to the conclusion that there was no longer confidence that the final commissioning of the signalling equipment from Lewes to Seaford could be completed over the four-day closure of the railway.
“We were not prepared to risk the lines not reopening for passengers on Monday morning and have therefore taken the very difficult decision to postpone that part of the work.
“Unfortunately, when we learned of the risks to the final commissioning it was too late to reinstate a train service, so we are doing as much of the signalling and other planned work as possible over the four days, as well as bringing forward additional planned work to ensure we make best use of the closures and minimise future disruption for passengers.
“We’d like to apologise to passengers for not delivering all the benefits of the upgrade over the closure as promised. We’d also like to apologise to our Southern colleagues at Govia Thameslink Railway who have worked closely with us to plan these closures and who will be running the bus replacement service over the four days.
“We will now work with our partners and local stakeholders to reschedule the final commissioning to a date when it will cause least disruption to passengers, business and the local tourism industry.”