London Underground drivers are to stage another 24-hour strike in a dispute over the new all-night Tube.
Members of Aslef will walk out at 9.30pm on August 5, threatening fresh disruption to services.
The decision, by the union's executive, came as b oth sides in the dispute were invited to hold fresh talks in a bid to avert a repeat of last week's stoppage.
Members of four trade unions staged a 24 hour strike from last Wednesday evening, which crippled services until Friday morning.
A spokesman for the conciliation service Acas said: "Acas has written to London Underground, Aslef, RMT, TSSA and Unite, inviting them to attend exploratory talks tomorrow morning."
Unions are unhappy about pay and rosters for the new service, which is due to start on September 12.
The RMT, TSSA and Unite have yet to announce their next move on the industrial action front.
Steve Griffiths, London Underground's chief operating officer, said: "We have made a very fair and reasonable offer on pay and the introduction of the night Tube and we continue to urge the trade unions to put it to their members.
"We will be at Acas tomorrow for further discussions to resolve this issue and trust the trade unions will be too."
Labour's London Assembly transport spokeswoman Val Shawcross said: "Last week's Tube strike was deeply disruptive to many Londoners. What made the situation worse was that the mayor could have avoided it had he engaged and properly negotiated with staff.
"With another strike on the horizon it's time for Boris Johnson to tone down the rhetoric and start focusing on resolving this dispute before Londoners are forced to endure another day of travel chaos.
"The fact is that everyone wants the night Tube to work, it's just got to be done right. Londoners elected Boris to lead the capital, that means talking to the unions and finding a resolution, not sitting on the sidelines throwing insults around."
John Allan, chairman of the Federation of Small Businesses, said: "This second Tube strike in less than a month will be disappointing news and another blow for businesses.
"Many small companies proved their flexibility and resourcefulness last week, allowing their employees to work remotely, or hold meetings with clients online rather than in person.
"Nevertheless, there will be further damage to the economy and millions of pounds lost to businesses if this strike goes ahead."