Passengers have been warned to expect disruption to their journeys because of two 48-hour strikes by London Underground (LU) workers over the closure of Tube ticket offices.
The company announced contingency plans for dealing with the walkouts from 9.30pm on February 4 and 11 by members of the Rail, Maritime and Transport (RMT) union and the Transport Salaried Staffs Association.
Transport for London (TfL) said there will be disruption to journeys on Wednesday and Thursday next week, while buses and other modes of transport will be very busy.
Talks to avert the strikes are being held, but TfL set out a range of measures it said would help keep London moving if the industrial action goes ahead.
TfL said Tube services will be disrupted from Tuesday evening so customers were urged to try to complete journeys as early as possible.
Tube services will not return to normal until Friday morning.
Extra bus and river services will be laid on, and so-called ambassadors will be on duty to keep Tube stations open and provide travel information and advice.
TfL is writing to hundreds of businesses across London to ask them to be flexible and make it easy for staff to cycle to work, and to allow them to wear casual clothes on the day.
LU is planning to close every Tube ticket office, with the loss of hundreds of jobs, arguing that few passengers use them.
Managing director Mike Brown said: "We're committed to implement these changes without compulsory redundancies and there'll be a job for everyone at LU who wants to work for us and be flexible.
"I urge the leaderships of the RMT and TSSA to work with us to help shape the future of the Tube, rather than threatening Londoners with unnecessary disruption.
"All a strike will achieve is lose those who take part four days' pay. Should the strike go ahead, we'll do all we can to keep our customers informed and ensure we keep London moving and open for business."
RMT leader Bob Crow said: "Instead of trying to con Londoners into believing that Tube chiefs can either break the strike or damp down its impact, the mayor and his officials should be focusing their efforts on reaching a negotiated settlement to a dispute which is about maintaining safe staffing levels, quality of service and access for all.
"RMT is once again appalled that TfL are trying to fool the public into believing that volunteer 'ambassadors' can run a safe service as an alternative to experienced and highly trained Tube staff.
"That is dangerous and provocative nonsense with potentially lethal implications and the union will be alerting Tube safety authorities of our serious concerns over this cavalier disregard for normal safety rules."
Manual Cortes, leader of the TSSA, said: "This latest statement from the mayor's team proves what we have been saying all week at Acas - he is not serious about a negotiating a sensible settlement to ensure the safety and security of the Tube in the future.
"Instead of talking about an army of volunteers to beat the strike, he should be sitting down with us to reach a deal by next Tuesday which would allow us to call off the action."