Maintenance workers on London Underground are starting a 72-hour strike today in a row over pensions which union leaders believe will hit services.
Members of the RMT union who maintain and upgrade the Jubilee, Northern and Piccadilly lines and deal with emergencies across the network walk out at 4pm until the same time on Friday.
The union said the action would disrupt services across the system, but London Underground denied this, maintaining there would not be any significant impact.
Talks aimed at resolving the dispute 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.
"The RMT's ballot showed that less than 44% of its members voted in favour of strike action. We urge the RMT leadership to return to discussions, and we remain ready to continue such discussions at any time to try and avert the strike.
"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."
RMT General Secretary Bob Crow said a 4-1 vote in favour of strikes was a "massive mandate", adding: "This dispute is about justice and about ensuring that all groups of staff under the umbrella of London Underground receive the same rights and benefits.
"We have gone through all the negotiating channels on both these issues but Tube Lines has refused to budge and have refused to engage in meaningful talks and that pig-headed approach has raised the temperature on the shop floor and is reflected in this result.
"Former Metronet employees have been allowed back into the TfL Pension Fund and now also have the same travel facilities as other LU employees.
"Tube Lines is now a wholly owned subsidiary of Transport for London and there is simply no excuse for refusing to give equal pension and pass rights."
The RMT also claimed that the dispute indicated London Underground was planning to reintroduce Public Private Partnerships. The two private firms, Tube Lines and Metronet, originally signed deals to maintain the Tube network but London Underground then had to take back control after the firms ran into problems.
However, LU's Managing Director Mike Brown denied any such plans existed. He added the Jubilee Line upgrade had only been successfully completed after Transport for London took over from Tube Lines.