Further talks to avert a Tube strike by thousands of Underground workers were being held on Wednesday.
Members of the RMT and TSSA are due to stage a 24-hour walkout from Sunday evening.
The conciliation service Acas will host the talks between London Underground and the two unions, whose members have been banning overtime.
TSSA general secretary Manuel Cortes said he was hoping to reach agreement with the system's management to restore the number of Tube employees to "safe" staffing levels.
Mr Cortes said: "This dispute has been rumbling on since Boris Johnson, in his last act of wanton, City Hall vandalism, decided to axe 800 jobs on the Tube, close ticket offices and play a perilous game of Russian roulette by downgrading safety practices.
"Talks so far have been taking steps in the right direction. Everyone round the table agrees more staff must be taken back on the Tube.
"But the talks are too slow for my members who had great hopes in a new Labour administration quickly undoing the damage done by Johnson and the Tories."
Steve Griffiths, Chief Operating Officer for London Underground, said extra station staff were being recruited following previous talks.
He said: “We will continue working with the unions as well as implementing the independent recommendations made by the London TravelWatch review.
“We would encourage the unions to work with us on this process rather than threatening strike action.”
