Commuters have been warned to brace themselves for travel "chaos" as a walkout on Southern rail coincides with a strike by London Underground drivers.
Services will be hit from Tuesday in long-running disputes over issues that include staffing and safety.
Southern said the action was a "co-ordinated and cynical manoeuvre by the unions".
The RMT union said "gross mismanagement of this basket case franchise" was to blame.
Strike and industrial actions:
- RMT members on Southern will strike for three days from Tuesday in protest at changes to the role of conductors.
- The network will also be hit by an overtime ban by drivers' union Aslef ahead of strikes later in December and the new year.
- Meanwhile, RMT members on London Underground's Piccadilly, Hammersmith and City and Circle Lines will strike for 24 hours from Tuesday evening.
Southern warned passengers to expect "severe and significant" disruption each day from Tuesday.
It said Aslef's continuous drivers' overtime ban would severely affect services every day.
On RMT strike days around 50% of the full timetable will operate but if the Aslef strikes go ahead, no Southern services will run.
Its owner, Govia Thameslink Railway, is taking legal action to try to stop the strikes.
Southern director Alex Foulds said the action was "wholly unnecessary and unjustified".
He added: "Customers are advised that stations will be incredibly busy.
"If passengers can make alternative travel arrangements they should, and if they don't have to travel they shouldn't."
Southern rail strike dates:
00:01 Tuesday 6 December to 23:59 Thursday 8 December (RMT)
00:01: Tuesday 13 December to 23:59 Wednesday 14 December (Aslef)
00:01 Friday 16 December to 23:59: Friday 16 December (Aslef)
00:01 Monday 19 December to 23:59 Tuesday 20 December (RMT)
00:01 Saturday 31 December to 23:59 Monday 2 January (RMT)
00:01 Monday 9 January to 23:59: Saturday 14 January (Aslef)
Thameslink is not directly affected by the strike action, although its services are expected to be extremely busy.
London Underground said passengers should plan for no Piccadilly line service and a significantly reduced service on the Circle and Hammersmith & City lines from Tuesday evening until the end of Wednesday.
Steve Griffiths, LU chief operating officer, urged the RMT to "work with us constructively on the issues it has raised rather than disrupt our customers with strikes."
Mick Cash, general secretary of the RMT, said: "Members have been warning for years of deep-seated problems on the Piccadilly line... Our driver members are forced to strike as they are at the end of their tether."
TSSA staff working on the Tube will refuse to work overtime from Thursday in a separate dispute over jobs and ticket office closures.