A 72-hour strike by London Underground (LU) staff will start later in row over pensions and benefits.
Workers who maintain and upgrade three lines on the Tube are to walkout from 16:00 BST.
The Rail, Maritime and Transport union said the action will disrupt services across the system, but LU said there would not be any significant impact.
The union wants employees of Tube Lines to join Transport for London's pension scheme.
Talks aimed at resolving the dispute, involving workers on the Jubilee, Northern and Piccadilly lines, ended without agreement at the conciliation service Acas last week.
Jon Lamonte, Tube Lines' chief executive, said: “Since becoming part of Transport for London all Tube Lines' employees have kept their existing terms and conditions of employment.
“Should this industrial action go ahead, we expect it to have no significant effect on Tube services.
“We have well-practised contingency plans in place to ensure that the essential maintenance required on the railway can either go ahead as planned or can be rearranged.
“We do not and will not put passenger or staff safety at risk.”
via BBC News