Drivers on the London Underground are set to stage a strike on 6 and 7 December over ongoing disputes. The strike will effect the Piccadilly, and Hammersmith and City lines.
The walkout looks set to coincide with a strike by workers on the Southern rail train network and could cause travel chaos for millions of commuters.
Tube drivers for the RMT union previously voted "overwhelmingly" to take strike action after they failed to reach an agreement with Transport for London (TfL).
The union has now confirmed that drivers are to strike for 24 hours on 6 and 7 December in two separate disputes over a breakdown in industrial relations, breaches of procedures and bullying and harassment of staff.
The Piccadilly Line is the direct course for services towards Heathrow Airport, the busiest terminal in the UK.
RMT General Secretary Mick Cash said: "This dispute on the Hammersmith and City Line is about the basic issues of protecting working conditions of our members and defending agreements from attempts to drive a coach and horses through them. The management are out of control and the anger at their failure to follow procedures has boiled over.
"This breakdown in industrial relations should never have been allowed to happen and if agreements and processes had been adhered to from the off the package of issues at the heart of the dispute could have been resolved through the joint machinery.
"In the separate dispute involving drivers on the Piccadilly Line, safety is again a major factor and is tied in with the ripping up of policies and procedures and ignoring warnings from staff. Our members have been left exposed and vulnerable and we have no choice but to blow the whistle before lasting damage is done.
"The union remains available for talks in both disputes."
The strike will coincide with a similar walkout by guards on Southern rail due to take place between 6 and 8 December.