London Underground workers will stage a new walkout on 15 March over pensions, job losses and contracts.
Tube drivers represented by the Aslef union had already announced a strike for the same day, but will now be joined by members of the RMT.
The latest industrial action will fall on the day chancellor Jeremy Hunt is due to deliver his Budget to the House of Commons, with over 100,000 civil servants also expected to walkout over their pay and pensions dispute.
And it will come a day before other rail workers across the country carry out their own strikes over pay and conditions.
Read more: Who is striking and when?
General secretary Mick Lynch said the staff provided “an essential service to the capital, making sure the city can keep moving and work long hours in demanding roles”.
And he said they would “never accept job losses, attacks on their pensions or changes to working conditions in order to pay for a funding cut which is the government’s political decision”.
Mr Lynch added: “In return, they deserve decent pensions, job security and good working conditions and RMT will fight and tooth nail to make sure that’s what they get.”
The announcement comes amid a raft of industrial action across the public sector, with teachers beginning three days of strikes on Tuesday.
March will see further walkouts by teachers, ambulance workers, university staff and junior doctors.
But while some nurses will strike, those represented by the Royal College of Nursing have postponed their planned action while they hold negotiations with the government.
