Night Tube not deliverable on time, claims RMT union
Union bosses have warned there will be "chaos" if the Night Tube is launched in London as planned next month.
Members of the Rail, Maritime and Transport Union (RMT), will strike for a second time this week in a row over pay and conditions on the service.
Transport for London (TfL) stressed it was "focussed on 12 September" for the launch, but the RMT says bosses privately believe it is not possible.
Four unions have rejected London Underground's latest pay deal.
'Ain't gonna work'
"These tubes are going to be running for the best part of 72 hours," an RMT spokesman said.
"At the moment they're going to lose two engineering and maintenance shifts.
"These shifts are absolutely essential in terms of testing the infrastructure, cleaning the tunnels, getting rid of the dust. The only time they're going to have to fix these problems is overnight on Sunday. They haven't planned for any of this.
"Behind the scenes TfL managers are telling you this ain't gonna work."
The RMT said its members will be forced to work extra shifts to plug "gaping holes" in staff capacity.
The Unite union, the train drivers' union Aslef and the Transport Salaried Staffs Association (TSSA) have also rejected the latest offer from London Underground, which is designed to compensate staff for weekend night working.
This week's 24-hour strike will start at 21:30 BST on Wednesday, although commuters travelling home during the evening rush hour are expected to face delays.