RMT leader Bob Crow says: "Our EDF Powerlink members are furious at the failure of the company to make a meaningful pay offer. Instead of entering into serious talks they have run to the courts and have wheeled out the anti-union laws in an attempt to shackle RMT. Unless we receive a positive offer this action will go ahead and the consequences for the Underground will be serious."
Transport for London say they're confident that "this action does not present a significant risk to the operation of the Underground."
However, Unite members have been balloted and may be taking the same action as RMT members at the same times.
There is also a separate dispute with LUL Willesden Green Group Station Staff over alleged victimisation of a staff member. "The union has therefore decided to use the mandate for industrial action and our members on the group have been instructed not to book on for any shifts that commence between 04:59 and 23:59 on Wednesday 23rd December 2009."
Update: 22nd December 2009 - Unite members will definitely be taking strike action from 8pm 22nd December 2009 until 8pm 27th December 2009. No updates from TfL as yet as to whether this will cause a disruption to services on the Tube. Although Unite say "EDF has failed to reach a compromise with Unite and now there is a high risk that the tube will be brought to a halt at the busiest time of year."