Go to the Transport for London website and you'll find this bold declaration: “In the early hours of 12 September we will launch our night time Tube service."

The Night Tube will support London's 24-hour lifestyle by offering a round-the-clock service on Fridays and Saturdays, on five lines – the Jubilee, Victoria, and most of the Central, Northern and Piccadilly.

The unions may well have undermined the first seven words of that statement with the Mayor of London Boris Johnson backing away from a fixed launch date in the face of continuing industrial action.

It may be the strongest indication yet that a long, hot summer of unrest lies ahead for Londoners as the dispute becomes more fractious and the Mayor shows himself intent on casting the dispute as a “show of strength”.

The quartet of rail unions have now come together again to target August 5/6 as the next 24 hours of action in their dispute with TfL over extra hours and unsocial scheduling.

Aslef, RMT, Unite and TSSA left talks at conciliation service Acas on July 14 with little sign of a rapprochement forcing the Mr Johnson to broaden the launch to "autumn" and effect a lack of interest in any particular date.

At Mayor’s Question Time at the City Hall, member Richard Tracey pointed out the happy coincidence of the planned September 12 launch and the arrival of the Rugby World Cup three days later.

He said: “There will be a number of people who will be welcoming some extra transport facilities.”

But the Mayor opted to sidestep any celebration of the coincidence, simply saying: “I think it’s a great project for the city and we’ll get it through.

"I can’t say exactly what date it is and I don’t particularly care but we’ll get it through this autumn.”

Mr Johnson said it appeared as if the unions were not interested in reaching an agreement.

London Assembly Labour member Val Shawcross accused the Mayor of being unhelpful and divisive, while Mr Johnson challenged her and fellow party members to denounce the strike.

He listed the package on offer – a £500 bonus for all staff and a 2% rise plus a £2,000 one-off bonus for drivers on the five lines running night services.

He challenged Labour members again to make a case for TfL’s intransigence in light of the deal on the table. None took him up on his invitation.