Strike action due to hit thousands of passengers is set to start on Monday after South Western Railway (SWR) and the RMT union failed to reach a deal.
Hundreds of thousands of commuters on trains into and out of London's Waterloo station and towns and cities like Portsmouth, Reading and Southampton face disruption in a long-running dispute over guards on trains.
Members of the RMT (Rail, Maritime and Transport union) who work on the SWR will walk out for 27 days from 2 December, until New Year's Day.
It is the longest stretch of action against a major rail operator in living memory.
The rail firm says only around half of services will run, including those to and from Waterloo, the country's busiest railway station.
The strike will not take place on the day of the 12 December general election and as usual there will be no trains on Christmas Day and Boxing Day.
SWR said in a Twitter statement: "RMT strike starts tomorrow. Make sure you visit http://southwesternrailway.com/strike to plan your journey as we will be running an amended timetable until the New Year. We know this will make traveling more difficult and are working hard to keep you moving."
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Talks between the two sides collapsed last week, with the union and company blaming each other.
Mick Cash, RMT general secretary, said: "RMT is angry and frustrated that a set of proposals that would have guaranteed the safety critical role of the guard at the point of despatch, and which would have cost the company absolutely nothing, have been kicked back in our faces.
"There is no rational explanation for the company position and we can only assume that either they or their paymasters in Government want this strike action to go ahead for politically motivated purposes.
"Our action goes ahead from Monday in defence of passenger safety and accessibility, and the blame for that lies wholly with SWR and their wrecking strategy.