Workers who clean London Underground stations are striking over the new year in a dispute over pay.
Rail Maritime and Transport union members working for Initial and ISS began the 48-hour walkout at 5.30am this morning, and will continue until January 2.
A spokesman for ISS said: "A small number of employees have decided to take industrial action, as is their right.
"We are delivering a full service and the lines we take care of, the Jubilee, Northern and Piccadilly lines, will not be affected. We will continue to provide a safe environment for passengers and Tube workers alike."
General secretary Bob Crow said: "With the eyes of the world on the London new year celebrations, the many millions watching the countdown must not forget the London transport cleaners on minimum pay rates doing some of the dirtiest jobs in often appalling conditions with no proper recognition and reward whatsoever.
"It is these same staff who have played a key role in keeping services clean and safe who are now forced again by their employers to take action for a living wage and recognition of their efforts.
"It is time for London Mayor Boris Johnson to stop talking about a living wage and start acting to make sure that contractors on his services actually deliver it."