The RMT union warned it was pressing ahead with Tube strikes next week after talks in recent days with Transport for London failed to resolve a dispute over a four-day week.
New talks are set to take place on Monday between TfL and the RMT to try to avoid the industrial action on the Underground.
But the RMT appeared to be seeking to ratchet up pressure on transport chiefs to offer concessions.
An RMT spokesperson said: “Strike action by London Underground drivers next week is scheduled to go ahead following TfL’s continued refusal to engage meaningfully with the union’s concerns over the proposed compressed four-day working arrangements.
“Our members have raised serious concerns around fatigue, longer shifts, reduced flexibility and the impact these proposals could have in a safety-critical role.”
He added: “RMT remains available for meaningful talks, but London Underground cannot push ahead with changes to drivers’ working conditions while refusing to properly address legitimate safety and workplace concerns.”
The planned meeting on Monday is likely to raise hopes of an eleventh-hour breakthrough in the stand-off between TfL and the RMT over the proposed new four-day week for Tube drivers.
Underground drivers, who earn around £74,000 a year on average, are being offered a voluntary four-day week with no pay cut.
Claire Mann, chief operating officer at TfL, said: “We are disappointed to see the RMT continue with this industrial action.
“We still believe that the points they have raised can be worked out in time, through more detailed discussions and we are continuing to talk to the union’s representatives to find a way to avoid disruption to London.”
But if the latest talks are also unsuccessful then RMT Tube drivers will walk out on Tuesday June 2 and Thursday June 4 from 00.01 to 23.59.
Service are expected to run on most lines but disruption is likely.
TfL has stressed that there will be limited service before 06:30 and customers should complete their journeys by 21:00.
No service is expected on the Circle line, Piccadilly line, the Metropolitan line between Baker Street and Aldgate, and the Central line between White City and Liverpool Street.
Other services, including the Elizabeth line, tram, London Overground and DLR will run as scheduled but are expected to be busier than normal.
This round of strikes is during the 24 hours of each day, rather than straddling two days as happened in the industrial action in April.
But services on the network could also be hit the days after the walkouts due to the knock-on effect.
Strikes planned for earlier this month were called off by the RMT, which will leave hope for many Londoners that a similar step will be taken for the June walkouts.
The row centres on new working conditions for drivers which the RMT has dismissed as a “fake” four-day week, arguing that it is five days compressed into four.
It has also raised concerns over fatigue among drivers and late notice of shifts.
But its position has been significantly undermined by the Aslef train drivers’ union backing the deal and praising it as offering the best working conditions on the whole of the mainline rail network.
Aslef has stressed to its members that the reforms are the “biggest improvement in working conditions for Underground train drivers in decades”.
Key benefits from the changes, according to the union, include an extra 35 days away from work a year, and average weekly rostered hours being cut to 34 from current average of 36.
It will also be easier to arrange roster patterns, it added, to block leave together, or take long weekends or mini-breaks, and would save money on travel and childcare costs.
For the first time, drivers would be allowed to volunteer for overtime which would be paid at time and a quarter.
The level of support for the RMT action has also been questioned after more than half of services ran on the Underground network during the April walkouts.
Some RMT drivers turned up to work.
London Mayor Sir Sadiq Khan has been urging the RMT and TfL to get around the table to find a solution to the dispute.
Earlier this week, TfL gave the RMT new reassurances that the four-week offer would be voluntary and no driver would be forced to accept it.
Tube users are being urged to plan their journeys before they travel on strike days using London’s official travel app, TfL Go, or through the TfL website.
