A fresh 24-hour Tube strike is due to start from 18:30 BST following the failure of talks to resolve a dispute between unions and London Underground.
All stations are expected to close, with the four main Tube workers’ unions supporting the industrial action.
Drivers are unhappy at pay and conditions offered for working on the new night service.
London Underground (LU) apologised for the disruption, saying the latest offer aimed at improving work-life balance.
Tube stations across the capital will close for the second time, following a 24-hour walkout on 8-9 July.
Services will stop running from 18:30 BST on Wednesday and there will be no trains on Thursday. Transport for London (TfL) advised commuters to complete their journeys before the action begins.
Some 250 extra buses will be laid on during the strike and extra river services will also be run during peak hours, TfL said.
LU said the new offer included a 2% salary increase this year, an extra £200 per night shift for drivers and a £500 bonus for night Tube staff when the service is introduced in September.
LU also offered a further £500 payout by next February.
Aslef, the Rail, Maritime and Transport (RMT) union and the Transport Salaried Staffs Association (TSSA) have all turned down the offer.
Unite previously said its action “would go ahead”.
London mayor Boris Johnson said the unions needed to put the “very good offer” to their members.
He said: “It is crazy when you have the technology not to put a 24-hour service in a 24-hour city.
He added: “Conversations have been going on about this since October and what really staggers me is we are on the verge of industrial action again and I apologise to Londoners about the inconvenience.”
BBC transport correspondent Richard Westcott
The sticking point comes down to guarantees over weekend working and time off.
I’ll give you an example. The unions say that a new night shift will mean a driver finishing at 08:00 BST on Sunday and then possibly starting work again on Monday morning. So, the Sunday will count as their day off. They say that’s not enough rest time after a night shift.
To try to reassure staff, LU has just sent out draft copies of what new shift-patterns might look like. It says staff will either work the same number of weekends a year, or fewer. And in the end, anyone who doesn’t want to work nights won’t have to.
But these talks clearly haven’t been going well.
I understand the unions pulled out in the last couple of days (not permanently though). The relatively new team at LU is experienced in business but not in the railways, and I get the impression that there has been a bit of a culture clash over the way both sides are used to doing deals.
RMT general secretary Mick Cash claimed the night Tube plan was “rushed and botched from the off” and said his members would “not accept that their work-life balance should be wrecked to plug the gaping holes in staffing capacity that should have been dealt with from day one”.
Finn Brennan, from the train drivers union Aslef, said: “The big issue in this dispute has never been about money, the key point has always been to make sure change comes in through negotiation and agreement and isn’t just forced upon people.”
London Underground said they had offered an “extremely fair revised” deal and drivers would have the same number of weekends off and staff would not be asked to work more hours.
LU’s managing director Nick Brown said: “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.”
Questions have been raised over whether the dispute will be resolved in time for the night service to begin as planned on 12 September.
The RMT has also said it will ballot engineers working for Tube Lines for strikes over the night Tube dispute.
The Tube Lines engineers maintain the Piccadilly, Northern and Jubilee Lines.