A dispute about night Tubes in London which has led to a strike has worsened after a union announced it will ballot more workers for industrial action.
London Underground (LU) services will stop from this evening and will not resume until Friday morning.
Unions are unhappy at pay and conditions being offered for the new all night service due to start on September 12.
The RMT said it will now ballot engineers working for Tube Lines for strikes over the same dispute. The Tube Lines staff maintain the Piccadilly, Northern and Jubilee Lines. The company is part of the legacy from the collapsed public-private partnership (PPP) that the RMT said created "chaos" across LU.
General secretary Mick Cash said: "Tube Lines staff have been offered a deal over the next two years on pay and 24-hour running which mirrors that on offer to the rest of the LU workforce.
"It is as unacceptable on Tube Lines as it is across the rest of the combine and as a result these essential maintenance staff will now be balloted for both strike action and action short of a strike."
LU managing director Nick Brown apologised to passengers for the disruption and urged the four unions involved in the dispute to put the latest offer to their members.
"Drivers will have the same number of weekends off as now and no one will be asked to work more hours than they do today. Everybody will remain entitled to two days off in seven. Annual leave will remain at 43 days for a train driver and 52 days for station staff.
"The unions rejected this fair offer outright and instead demanded more money, the hiring of even more staff - including for ticket offices that customers no longer use - and a 32 hour, four day week. No employer can afford to meet those sorts of demands.
"We continue to urge them to call off the strike, put the new offer to their members and not subject Londoners to further unnecessary disruption. We remain available for talks at any time."
London mayor Boris Johnson also apologised to Londoners for the strike, admitting they would face "huge inconvenience".
He told BBC Radio 4's Today programme that the offer was "generous".