Maintenance staff on three tube lines are entering their third day of strikes today.
Workers on the Picadilly, Jubilee and Northern Lines began the industrial action earlier this week in a row over pensions.
Members of the Rail, Maritime and Transport Union went on strike at 4 o'clock on Tuesday afternoon and have vowed to stay at their pickets until the same time on Friday afternoon - a total walkout of 72 hours.
The workers maintain and upgrade the tube lines mentioned and also carry out emergency repairs across London's Tube network.
Yesterday leader of the RMT Bob Crow criticised the emergency safety teams who were acting in place of those on strike, saying they were ill equipped and inadequately trained.
London Underground strongly denied the safety concerns and said it would never do anything to compromise safety.
Baroness Jo Valentine, Chief Executive of business group London First, said:
Meanwhile yesterday it was announced London Underground cleaners are also to be balloted for strike action concerning pay, pensions and benefits.