Maintenance staff on three tube lines are entering their third day of strikes today.
A strike affecting the Jubilee, Northern and Piccadilly lines goes ahead from 4pm.
Union bosses have forecast "widespread" disruption, as workers who maintain and upgrade tube lines go on strike next week.
RMT General Secretary Bob Crow has accused the London Mayor Boris Johnson of going back on his election promises, as his union's members prepare to ballot for strike action.
The ballot opening today is over the threat to jobs across the combine resulting from the plans to close every single ticket office in the face of previous assurances and promises from the Mayor, before he was elected, that he would not close any.
Those planned staff cuts impact on every single member of tube staff both in terms of future employment prospects, recruitment freezes and in terms of the safe and efficient running of tube services
– Bob Crow, RMT Union
The reality of course is that any move to longer running hours would require more staff not less as maintenance schedules are compressed and stations are flooded with revellers rolling out of bars and nightclubs at three o'clock in the morning.
The idea that they would be piling into unstaffed stations controlled by a member of staff three stops down the line with an IPad is lethal nonsense.
Workers on London's tube network will vote on industrial actionlater.
RMT union members are considering strikes in the New Year after theMayor announced job cuts and ticket office closures.
Transport for London saysthe scheme will save £50 million pounds a year.
The Rail Maritime and Transport workers union has confirmed that a strike on London Overground services will go ahead this weekend after an agreement was not reached over job cuts.
The RMT claims that 130 guard jobs are at risk over plans to run driver-only services on some trains. They will walk out for 48 hours from 00:01 on Sunday August 25th.
The strike takes place over the weekend of Notting Hill Carnival and is expected to cause widespread disruptions to services, especially on the Richmond and Clapham Junction to Stratford Junction routes.
Transport for London said they were working hard to keep as many services running as possible and there will be alternative Tube, bus and rail services available.
London Overground has released a statement, following news that the RMT union is planning a series of strikes in protest at the removal of guards from its trains.
We are disappointed by the result of the ballot, but would highlight that fewer than half of our 124 conductors voted, and only 43% of those voted in favour of industrial action.
We believe industrial action is unnecessary. LOROL continues to give the RMT assurances on employing conductors in alternative customer service roles and offering a generous voluntary redundancy package to those who want it.
– Peter Austin, London Overground managing director
We urge the RMT to work with us to maintain progress in safeguarding jobs and avoid disruption to passengers.If industrial action goes ahead on Sunday August 25th and Monday August 26th we believe we will still be able to run a regular service on the majority of the London Overground network with alternative arrangements for the routes affected. We will update our customers when we have more information.
Rail union RMT has announced that guards on London Overground will strike on the following days:
Sunday 25th August
Monday 26th August
The strike is over a move by the company to introduce Driver Only trains (without guards) on the whole of the London Overground network.
Peter Austin, managing director for London Overground Rail Operations Limited (LOROL), said:
"We are disappointed by the result of the ballot, but would highlight that fewer than half of our 124 conductors voted, and only 43% of those voted in favour of industrial action.
"We believe industrial action is unnecessary. LOROL continues to give the RMT assurances on employing conductors in alternative customer service roles and offering a generous voluntary redundancy package to those who want it.
"We urge the RMT to work with us to maintain progress in safeguarding jobs and avoid disruption to passengers."
Guards on London Overground have voted for strike action in a row over jobs. Members of the Rail Maritime and Transport union backed walkouts by 9-1 as part of a campaign against cuts.
The union said 130 guards' jobs face being axed under plans to introduce driver-only operations on the network.
General secretary Bob Crow said:
"RMT have sent out a clear message in this ballot for both strike action and action short of a strike on London Overground over the appalling, cash-driven assault on our guards members and the absolutely essential role that they play.
"These are the very same staff who have been praised for safely evacuating passengers from emergency situations and who are the eyes and ears of the service at a time of growing violence and thefts on our trains."
Serco spokesman Andrew Hill said: "Together with our recognised trade union Community, we have already reached an agreement on the issues the RMT have raised around pay and working pattern changes.
"This was achieved through a comprehensive, thorough and fair consultation with staff that resulted in a pay rise of 2%, which is above the national average and included other improvements to their terms and conditions.
"This strike action is therefore wholly unnecessary. We will make every effort to ensure that we minimise its impact for the thousands of Londoners who rely on the scheme to get around the city every day."
RMT general secretary Bob Crow said:
"Despite talks with the company, we have not been able to make significant progress and so we now have no option but to announce strike dates this month."
Workers on the bike hire scheme in London are to stage a 48-hour strike next week in a row over pay and conditions.
Members of the Rail Maritime and Transport union employed by Serco on the so-called Boris Bikes - named after London Mayor Boris Johnson - will walk out from 9pm on the evening of Sunday 11 August to 8.59pm on Tuesday 13 August.
The workers, who move the bikes around the capital and maintain them, voted in favour of action over pay, shift patterns and other issues.
A Serco for Barclays Cycle Hire spokesperson said that following a ballot, the RMT union has decided to take industrial action against Serco at Barclays Cycle Hire by stopping overtime from 00.01 on Tuesday 16th July.
AndrewHill, Contract Director at Serco forBarclays Cycle Hire said:
“Together with our recognised trade union Community, we have reached an agreement on the issues the RMT have raised through a comprehensive and fair consultation with staff.
We are therefore very disappointed that the RMT has decided to take unnecessary industrial action over pay and working conditions by stopping staff working overtime.
We will make every effort to minimise possible disruption in service to the thousands of Londoners who rely on the scheme to get around the City every day."