Rail passengers on some of the country's busiest routes face a fresh bout of travel chaos for the next three days because of another strike in the bitter row over the role of conductors.
Members of the Rail, Maritime and Transport union will mount picket lines at the start of the action on Tuesday, with disruption set to continue until Friday morning.
Southern said it will run around 61% of its normal full timetable but warned there will be a restricted service, with many routes having fewer trains, and on some routes there will be no train service.
The union said it had received a legal challenge, just hours before the start of the walkout, but Southern said it was "considering all possible options".
RMT general secretary Mick Cash said: " The union intends to continue with the planned action and is examining the details of the paperwork."
A Southern spokesman said: "On Friday they told their conductor members to accept a deal, and then tomorrow they plan to strike against it.
"It is a situation which will leave our passengers baffled, and in that context we are presently considering all possible options to stop the strike.
"We have written to the RMT about the validity of the dispute in light of recent developments. We await their response and will consider all possible options to stop the strike and that includes a legal challenge."
In a message to RMT members, Mr Cash urged them to continue preparing for the strike, organise picket lines and protests.
The RMT said earlier that Charles Horton, chief executive of Govia Thameslink Railway, which owns Southern, has been closely involved in discussions at the Rail Safety and Standards Board (RSSB) over the role of conductors.
The union said it had obtained a leaked briefing memo showing Mr Horton's links to the board, which it claimed proved the organisation was not independent.
Southern denied the claims and insisted the board was independent.
The RSSB said it did not recognise the document referred to by the RMT, adding that sections released by the union were not from a leaked briefing memo, but had been selectively taken from a publicly available research report.
A statement said: "In the background section of the report is says that 'The Department for Transport's rail Value for Money study (McNulty 2011) recommended that DOO(P)(driver-only operation) should become the default position for all services on the GB rail network' and does clearly mention safety.
"This study was commissioned by RSSB on behalf of the Rail Delivery Group to determine how DOO(P) could be implemented across the remaining 70% of the passenger network cost-effectively, without compromising current operational safety.
"Charles Horton is one of our non-executive directors but was not involved in the research project which has been quoted in the document published by the RMT.
"RSSB does not create or advocate any policies, but provides independent insight and guidance to the rail industry based on robust research and analysis, which can include economic analysis."
Paul Plummer, chief executive of The Rail Delivery Group, representing train operators and Network Rail, said: "The Rail Safety and Standards Board is an independent body and its valuable work and research has helped to ensure that Britain has Europe's safest railway.
"For more than 30 years thousands of trains have run safely every day across Britain with only the driver operating the doors. RSSB safety experts say that this is safe. The rail industry must modernise to deliver the better services passengers expect and deserve."