Tube passengers are being warned of disruption to services because of strikes by drivers.
Members of Aslef will walk out on April 8 and May 4 in a dispute over terms and conditions.
Transport for London (TfL) urged people to check before travelling.
On Monday April 8 severe disruption is expected, with little or no service on some lines, and Tube services will start later than normal on Tuesday April 9.
There will also be severe disruption on Saturday May 4, with little or no service expected on some lines, and Tube services will start later than normal on Sunday May 5.
Nick Dent, director of customer operations for London Underground, said: “We are encouraging Aslef to continue engaging with us so the strike action can be avoided.
“We would like to advise anyone travelling to please check before you travel, expect services to be busy and please allow extra time to complete your journeys.
“Strike action is disruptive, and TfL continues to meet the Aslef union at the conciliation service Acas this week in the hope that this strike can be called off.”
Finn Brennan, Aslef’s full-time organiser on London Underground, said: “Tube train drivers will strike in April and May in a long-running dispute over London Underground’s failure to give assurances that changes to our members’ terms and conditions will not be imposed without agreement and that all existing agreements will be honoured.
“Despite a previous commitment to withdraw plans for massive changes to drivers’ working conditions, London Underground management has established a full-time team of managers preparing to impose their plans.
“They want drivers to work longer shifts, spending up to 25% more time in the cab, and to remove all current working agreements in the name of ‘flexibility and efficiency’.”