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The threat of Christmas strike chaos on the Tube increased today as 1,600 ticket staff prepared to ballot for industrial action in protest over assaults by passengers.

They will join 4,000 station staff already set to vote on a walkout in a separate dispute over staff cuts.

Both ballots are expected to start later this month, with walkouts threatened in the run-up to Christmas after union leaders forecast overwhelming support for the strike calls.

Manuel Cortes, leader of the TSSA union, said the strike ballot was now “inevitable”, claiming London Underground had failed to act and protect staff from physical and verbal assaults.

Ticket offices were closed and staff moved out onto station concourses and platforms under former mayor Boris Johnson’s regime with claims it would improve service for passengers.

The union, as revealed by the Evening Standard last week, says it has logged more than 2,000 separate incidents and that some staff are “afraid” to go to work.

Mr Cortes said the plan “just hasn’t worked” and called for ticket offices to be reopened.

He said ticket machines either break down or cannot cope with demand during peak periods, with frustrated passengers then taking out their anger on staff.

“There’s been a sharp spike in staff feeling physically intimidated by frustrated since April. Staff have been assaulted and robbed as they stand at unprotected gate lines,” said Mr Cortes.

“Ticket machines are completely useless multi-taskers. They don’t give change, replace lost or broken Oyster cards, speak slowly to tourists or other people in need of human assistance,” he said.

LU chief operating officer Steve Griffiths said: “Everyone has the right to go about their work without fear or intimidation and we do not tolerate any form of verbal or physical assault on our staff.

“Our campaign against workplace violence sends a clear message that any assaults on staff will be fully investigated and we work closely with the British Transport Police to bring perpetrators to justice.”