Strikes by Southern Railway train drivers will now hit the company at the end of January, as well as next week, causing fresh misery for hundreds of thousands of passengers.
Members of Aslef were due to walk out for six days from Monday in a bitter dispute over driver-only trains.
The union changed the dates to January 10, 11 and 13, but announced three fresh strikes on January 24, 25 and 27.
Southern said no trains will run on the three strike days next week, but services will operate on the two non-strike days although these will be hit by the continuing ban on overtime by drivers' union Aslef.
Aslef leader Mick Whelan said next week's action was changed out of consideration for how long the dispute will last, and because of concerns from the public about the impact of a week-long stoppage.
Southern's passengers have suffered months of disruption because of industrial action, staff shortages and other problems.
Aslef members are currently banning overtime which is leading to services being cancelled or delayed every day.
Mr Whelan said: "We are taking a longer-term view of this trade dispute. The company has not been prepared to move - it is simply going through the motions, turning up at Acas and telling us that it intends to impose driver-only operation.
"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."
Southern announced it is putting in place a number of alternative measures to try to help those commuters who have essential travel needs.
The train operator is organising 200 coaches or buses each day to provide road links for essential travel from nine Southern stations into nearby neighbouring train networks where they can connect into other train operators' services.
Southern advised people to work from home or remotely if they can, stagger journeys if possible and to only travel if it is "essential" and allow plenty of extra time for journeys.
Angie Doll, Southern's passenger service director, said: "There will be significant disruption and hardship next week caused by these pointless and unnecessary strikes. With this package of measures, we are putting in place a very limited number of alternative options to help people with essential travel needs get where they need to be.
"Unfortunately, there is no practical way we could replace 2,000-plus trains we run each day with buses, but what we are doing is providing transport to link some passengers into other operators' stations where services are running normally."
Rail minister Paul Maynard said: "All Aslef are doing is extending the misery for passengers. I urge Aslef to call off these wholly unnecessary strikes and come to the table for talks.
"This modern way of running trains has been safely used elsewhere in the UK for 30 years. There is no safety issue; the independent rail regulator has confirmed it is safe."
The Rail, Maritime and Transport union is also embroiled in a dispute with Southern over changes to the role of conductors, which has led to a series of strikes.