Bob Crow, the general secretary of the Rail Maritime and Transport Union, said: "RMT members have delivered a massive mandate for action following a straightforward demand for parity with other Tube staff.
"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, and our members have no choice but to strike to secure those basic rights.
"We have gone through all the negotiating channels 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 has resulted in this strike action being called.
"Former Metronet employees have been allowed back into the Transport for London (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 union remains ready to talk, and the strong mandate for this industrial action shows Tube Lines and TfL the depth of anger there is over this blatant lack of fairness and justice."
Maintenance staff on three tube lines are entering their third day of strikes today.
A strike affecting the Jubilee, Northern and Piccadilly lines goes ahead from 4pm.
Union bosses have forecast "widespread" disruption, as workers who maintain and upgrade tube lines go on strike next week.