London Underground drivers will stage a series of four strikes in coming weeks in a row over the sacking of a colleague, the RMT union has said.
The walkouts between June 19 and July 1 are likely to cause travel chaos for millions of commuters and tourists.
The strikes are over a continuing dispute involving the sacking of Northern Line driver Arwyn Thomas, whose claim of unfair dismissal has been heard by an employment tribunal, with the result expected in the coming weeks.
London Underground said it was "mystifying" that strikes were being threatened before the tribunal's decision.
The union's general secretary Bob Crow said: "RMT has made every possible effort to get Arwyn Thomas back to work and it is the intransigence of LU management, who have dragged their heels and failed to reach agreement over the past month, that has left us with no choice to put this strike action on."
A Transport for London spokesman: "We committed in good faith to legal discussions ahead of Mr Thomas' tribunal.
"Unfortunately, no agreement was reached. We have always made clear to the RMT that we would respond appropriately to the tribunal's finding."
The RMT claimed that Mr Thomas, a long-standing union activist and driver based at Morden, was sacked on "trumped-up" disciplinary charges following allegations made against him during a previous strike.
Drivers belonging to the RMT will strike between 9.01pm on June 19 and 3am on June 20; between 9.01pm on June 27 and 11.59am on June 28; between noon on June 29 and 11.59am on June 30; and between noon and 9pm on July 1.