Passengers are facing a twin threat of travel disruption because of two separate disputes involving Tube workers.
Members of the drivers' union Aslef working on London Underground's Piccadilly line are due to stage a 24-hour strike next Thursday over the sacking of a colleague, with further dates to be announced.
In another row over jobs and conditions, members of the Rail, Maritime and Transport (RMT) union based at a control centre in west London are set to stage walkouts from July 1-4 and ban overtime from July 8-15.
RMT leader Bob Crow said: "This group of workers are absolutely key to delivering the service on the Piccadilly line and sub-surface railways and the way that they have been treated by the management at a time when the system is under intense pressure is nothing short of shocking."
Phil O'Hare, LU's Piccadilly line general manager, said: "It is disappointing that Aslef has threatened to take strike action on the Piccadilly line, especially when only a third of those balloted have voted for industrial action.
"This train operator was dismissed for gross misconduct after knowingly passing a signal at danger and then proceeding with passengers on board without authority, without knowing the cause of the danger signal, and without notifying service control.
"The train operator has lodged an employment tribunal complaint, which is the appropriate way for this matter to be dealt with, not through threats of industrial action in defence of behaviour that Aslef itself does not condone."
Nigel Holness, LU operations director, said of the RMT dispute: "If this threatened strike were to take place we would expect to run a substantial amount of service on the affected lines and a normal service on the rest. Less than 30% of RMT members balloted were in favour of industrial action.
"We have been in discussion with the unions across the entire Tube network, including the affected lines, since 2007 on the impact of changes arising from our upgrading of signalling systems. We have already addressed nearly all of the issues raised by the RMT.
"There is absolutely no need for strike action over changes which would not be implemented for five years and which will be discussed at Acas on Friday."