A Tube strike is still set to take place just weeks after a staff walk-out disrupted London transport for 24 hours.
Members of ASLEF union plan to strike for 24 hours on August 5 but talks to resolve the industrial action are still under way.
An Acas spokesman said: "Talks resumed on Wednesday morning (July 22) with all the parties in relation to the current London Underground dispute"
Exploratory talks between ASLEF and Acas started on July 14 and are currently ongoing.
An ASLEF spokesman said: "London Underground aren't in a particular hurry as there's still some time. We've been talking all last week and talking again this week."
"Exploratory talks don't have to be finished until 24 hours before strike action is due.
"My best guess is that an agreement will be reached before then.
"We're edging slowly together towards resolution, London Underground don't want a strike at the moment and we don't either.
"We've always indicated that we are happy to talk, if we get an offer then we'll likely take it."
Tube drivers recently took industrial action, opposing the planned Night Tube due to come into effect in September.
Members of the RMT union walked out for 24 hours from 9.30pm on Wednesday July 8, through to the evening of Thursday July 9, causing chaos for commuters and residents.