Southern Railway has set a deadline for the Rail, Maritime, and Transport Union to accept an offer aimed at averting strikes.
The union has until Thursday to accept the operator's conditions, or it will terminate conductors' contracts and press ahead with changes to their role.
Southern Railway has had to deal with long-term strike action after the RMT staged a series of walkouts in recent months in protest at changes to the role of conductors on the operator's service, managed by Govia Thameslink Railway.
The latest strikes will start on October 11 and will be held intermittently until December 8.
More information on this as it develops.