A five-day strike has begun on the Southern rail network, during which just 60% of its normal timetable will run.
Southern and the Rail, Maritime and Transport Workers union (RMT) are involved in a dispute over the role of guards and talks to resolve the situation broke down last week.
When is the strike?
The strike will take place from Monday 8 to Friday 12 August.
Southern said in a statement: "Southern expects to run almost 60% of its normal timetable (or two-thirds of the temporary revised timetable) from Monday 8 August to Friday 12 August inclusive.
"There will be extra staff at stations to help passengers and arrangements in place for passengers to use alternative transport providers."
What has been said by Southern and the RMT?
Govia Thameslink Railway passenger service director, Angie Doll, said: "We are sorry that our passengers once again look set to suffer further disruption because of the RMT. The union must call this action off by Thursday afternoon if we are to avoid having to put in Monday's strike timetable."
RMT general secretary, Mick Cash, said: "This action has been forced on us by the arrogance and inaction of Govia Thameslink and the Government who have made it clear that they have no interest in resolving this dispute or in tackling the daily chaos on Southern.
"Our fight is with the company and the Government who have dragged this franchise into total meltdown. We share the anger and frustration of passengers and we cannot sit back while jobs and safety are compromised on these dangerously-overcrowded trains."