The London Underground and Rail, Maritime and Transport are holding last-ditch talks just hours before a three day strike, that could affect millions of people across London and surrounding areas.
Conciliation service Acas said the talks between the RMT and London Underground started at 10am.
RMT union members are set to walk out of their jobs for three days at 9pm this evening, in a long-running dispute with the London Underground over ticket office closures.
A two-day strike last week caused huge disruption, with roads clogged as people switched to other forms of transport.
A rail company is to take legal action against a trade union over a dispute which is disrupting its services.
Members of the Rail Maritime and Transport union (RMT) at East Midlands Trains were banned overtime a week ago, leading to timetable changes and cancellations this weekend.
The union said there had been a breakdown in industrial relations over a number of issues, including the scheduling of work in Nottingham.
Planned strikes by ScotRail workers on Saturday and Christmas Eve have been called off, the RMT union has announced.
Cleaners on London Underground will launch a 48-hour strike today in a dispute over an Olympic bonus payment.
Members of the Rail Maritime and Transport union employed by private contractors will walk out at 5.30am, with picket lines outside stations including Stratford, close to the Olympic Park.
RMT general secretary Bob Crow said:
"With the eyes of the world on the London Olympics the many millions enjoying the events around the globe must not forget the London transport cleaners on minimum pay rates doing some of the dirtiest jobs in often appalling conditions with no Olympics recognition and reward whatsoever."
Leader of the RMT Bob Crow said the tube strike will go ahead after talks between London Underground bosses and the union failed to reach an agreement over pensions.
He said: "The action next week remains on and it is the management's intransigence that is responsible for the disruption travellers can expect."
No further talks between leaders of the RailMaritime and Transport (RMT) union and London Underground leaders have beenarranged, raising the threat of disruption to Tube services next week.
Earlier this week RMT officials said its members will walk out at 4pm on Tuesday until the same time on Friday.