The Government tonight warned that a series of planned strikes by thousands of London Underground workers in a row over jobs would be "seriously damaging" as passengers were braced for travel chaos in the coming months.
Members of the Rail Maritime and Transport (RMT) union and the Transport Salaried Staffs Association (TSSA) will walk out for 24 hours from 5pm on September 6, the day many people return to work after the summer holidays and the House of Commons resumes following the recess.
Maintenance and engineering staff will be involved in the first stoppage, with similar 24-hour stoppages planned from 5pm on October 3, November 2 and November 28.
Other workers, including Tube drivers, signallers and station staff, will strike for 24 hours from 9pm on the same four days, threatening disruption in the evening of the walkouts as well as the following days.
The unions said up to 10,000 workers will be involved in the action, which will include an indefinite overtime ban from September 6, following votes in favour of strikes over plans to cut 800 jobs among station staff.
Transport Secretary Philip Hammond said: "A Tube strike will be bad for passengers, bad for business and bad for London.
"At a time when public finances are under pressure, any strike by Tube workers will be seriously damaging - undermining the case we are making within the spending review for continued investment in the Tube."
Transport for London urged both unions to call off the strikes, saying that staffing changes were needed because of the success of the Oyster pre-paid card system, which had led to some ticket offices selling less than 10 tickets an hour
Howard Collins, London Underground's chief operating officer, said: "It is clear that passengers can be better served by getting staff out from behind the windows of under-used ticket offices.
"We need to change, but we will do so without compromising safety, without compulsory redundancies, and in a way that means all stations will continue to be staffed at all times and all stations with a ticket office will continue to have one.
"The weak mandate for strike action, which saw only around 35% of TSSA members and less than a third of RMT members voting for a walkout, should resonate with the unions' leadership. These threatened strikes are in nobody's interest, and should not go ahead."
RMT general secretary Bob Crow said: "London Underground and Mayor Boris Johnson must understand that the cuts they want to impose are unacceptable to our members and will undermine safety and service for the travelling public.
"The Mayor was elected on a promise of maintaining safe staffing levels and he is doing the opposite, planning to leave stations and platforms dangerously understaffed and threatening to turn the network into a muggers' paradise.
"We have already had potential disasters narrowly averted, with fires at Euston and Oxford Circus and a runaway train on the Northern Line, and Boris Johnson's planned cuts would deal a potentially fatal blow to the ability to deal with emergencies."
TSSA general secretary Gerry Doherty said: "Boris Johnson may be prepared to go into the Olympic Games with a second class Tube service when the eyes of the world will be on the capital - we are not.
"We will defend a vital public service on which millions of people depend every day of their working lives. We will not see jobs and services sacrificed to pay for the sins of the City of London and Wall Street."
Richard Tracey, London Assembly Conservative Group transport spokesman, said: "The unions' ability to bring London to a standstill would be massively curtailed if the Mayor began the long-term process to introduce a driverless Tube."
The London Chamber of Commerce said each day the Underground was closed would cost London's economy £48 million and hamper recovery from recession.