Three new strike dates have been announced for Southern rail at the end of the month, Aslef, the train drivers' union, has announced.
Industrial action is planned for January 24, 25 and 27.
The announcement came shortly after the union said it would reduce next week's six-day driver strike to three days as it takes a "longer-term" view of the ongoing dispute.
Next week's strike was set to kick off on Monday, but it is now slated for Tuesday, Wednesday and Friday. The ban on overtime will continue throughout.
The dispute over driver operated doors has been in deadlock as the rail firm and government look to push through new measures that the unions say would impact safety and lead to job losses.
"We remain committed to a negotiated settlement, as was reached with ScotRail, but it is difficult to negotiate with people who are not prepared to be flexible," said Mick Whelan, general secretary of the union.
"We still believe a deal can be done but we are, at the moment, a long way from that position. It is time for the company to come up with a genuine offer rather than carry on posturing."
A spokesperson for Govia Thameslink Railway (GTR), Southern rail's owners, said the change in strike action was a "cynical ploy" to maximise disruption for passengers while minimising the impact on Aslef's driver's pay packets.
"Aslef's move shows pure contempt for the travelling public and it still causes massive disruption over next week," the spokesperson said.
"These strikes are pointless and they should call the whole thing off and let common sense prevail."
GTR has advised commuters to travel only if it's "essential", adding it will provide 200 extra coaches and buses linking nine Southern stations to nearby networks running Southeastern, South West and Gatwick Express trains.
"These buses will run from a small number of stations and space will be very limited, so we are asking those who don't need to travel, or who have a viable alternative, to leave the space for those who have no other option," said Angie Doll, Southern's passenger service director.
"Anyone who does travel should expect to queue, plan for longer journeys and realise the service they join will be exceptionally busy."