Several ongoing disputes continue to threaten the network

A series of rows over pay, the sacking of a driver and claims of harassment have contributed to the strike threat.

The RMT union has warned it plans to ballot 6,500 workers over industrial action unless an "acceptable" pay offer is tabled by London Underground.

Union leaders said there was a "massive gulf" between workers and managers.

Pay deal

General Secretary of the RMT union Bob Crow said: "From the start we have told LU that we would prefer a one-year deal.

"We're prepared to negotiate around that, but LU are insisting on a rigid four-year deal that our members told us they will not accept."

The latest offer includes a 4% pay rise this year followed by increases above the rate of inflation in subsequent years.

Jubilee line

The union also announced it will ballot Tube drivers on the Jubilee Line over the "blatantly unfair" sacking of driver Raj Nathvani who reported over-running a signal.

Mr Nathvani was sacked "without mercy" and a union official dealing with the case now faced dismissal, according to the RMT.

The union is also balloting 80 station staff at Canary Wharf Station over a "catalogue" of issues, including claims of harassment and victimisation of union reps and discrimination against gay workers.