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Millions of Tube commuters face the threat of major strike disruption next month called in protest over a train driver sacked after he allegedly failed an alcohol breath test.

Furious Tube bosses today condemned the RMT union which has ordered a strike ballot across all lines to begin next Monday as asking their members “to defend the indefensible.”

Walkouts could take place from the middle of next month.

The driver was stopped at his Northern line Morden depot before going on duty.

Nigel Holness, London Underground’s operations director for the Jubilee, Northern and Piccadilly lines, said: “Had he taken control of a train as he intended to that day, the individual’s choice to consume alcohol prior to commencing work put his customers’ and colleagues’ lives in danger.”

LU said the union had lodged official notification that the new ballot "on all Tube lines" will begin next Monday, 26 January, with the result on Tuesday, 10 February.

By law the union must then give seven days notice of any industrial action meaning 24-hour stoppages could begin from Tuesday, 17 February.

Union officials expect support for the strike call.

The ballot would only involve RMT train drivers - representing just over one third of the network's 3,200 drivers. The majority of the remainder belong to the rival union Aslef which is not involved in the dispute. Some Aslef members could, however, refuse to cross picket lines.

In an internal bulletin LU today made a pre-emptive plea to staff not to back the strike.

The bulletin, seen by the Evening Standard, states; “The RMT’s needless strike action will see its members sacrifice pay to support one RMT member who has admitted that he came to work – to operate a busy Northern line train – having consumed alcohol.”

The RMT staged a 24-stoppage by its drivers on the Northern line last month in protest over the dismissal - now they plan to escalate the walkouts across the entire Tube network.

Mr Holness said: "For the safety of our customers and staff we have a zero tolerance towards drugs and alcohol."

He said the employee - who was tested twice in front of "medical specialists" - was subject to a random inspection in June last year before going on duty.

The driver was one of several to take part in the random testing. Mr Holness said: “Only one employee failed and he did so twice; both the primary and secondary evidential tests.

“He subsequently conceded that he had consumed alcohol prior to his shift.

“I want to make clear that we have a legal obligation to protect our customers and employees in this way and we will not compromise this duty of care.”

He robustly denied allegations by the RMT that the breath test result had been “skewed” by a medical condition.